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Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
Plc scada for automation process control
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Plc scada for automation process control
Plc scada for automation process control
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Plc scada for automation process control

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PLC SCADA FOR AUTOMATION PROCESS CONTROL

PLC SCADA FOR AUTOMATION PROCESS CONTROL

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  • 1. PLC & SCADA for Automation & Process Control Dr. Mohammad H. Salah
  • 2. Introduction to Control Strategies Outlines Control Systems Continuous Control Systems Sequential (Logic) Control Systems Synchronous Control Systems Asynchronous Control Systems Mixed Synchronous/Asynchronous Control Systems Implementation of Synchronous Control Systems Relay Control systems Dr. Mohammad H. Salah Page no. 1
  • 3. Control Systems Any process contains the application (operative part) and control system (active coordinator). The best way to describe a control system is to use a block diagram. All control systems have, at least, three parts to them; An INPUT that takes information into the control system, A PROCESS that uses the input information to create the output information, An OUTPUT that passes information out of the control system. Control Systems - Inputs Input signals are provided by transducers / detectors that convert physical quantities into electrical signals. Depending on transducer used, the information detected can be discontinues (binary) or continuous (analog) representation of the input quantity. Output Quantity Binary Voltage Transducers Switch Measured Quantity Movement / Position Limit Switch Movement / Position Binary Voltage Thermostat Temperature Varying Voltage Thermocouple Temperature Varying Voltage Thermistor Temperature Varying Resistance Pressure / Movement Varying Resistance Light Varying Voltage Presence of Objects Varying Resistance Strain Gauge Photo Cell Proximity Cell Dr. Mohammad H. Salah Page no. 2
  • 4. Control Systems - Outputs Output devices (like relays, pumps, motors..) are tools used by a control system to alter certain key element or quantities within the process.they are also transducers but contrary signals from the control system into other necessary. There are also discontinuous (binary) or continuous (analog) devices Output Device Motor Quantity Produced Rational motion Input Electrical Pump Rational motion + product displacement Electrical Piston Linear motion / pressure Hydraulic / pneumatic Solenoid Linear motion / pressure Electrical Heater Heat Electrical Valve Orifice variation Electrical/Hydraulic/pneumatic Relay Elec. Switching / limited physical movement Electrical Control Systems The heater is an Open Loop control system. In this system the information from the output is not sent back to the input. However the room gets hot, the heater keeps producing heat until someone switches it off. If the heater had a thermostat, it would switch off by itself when the room reached a set temperature (the ‘input’ to the system). In this case information from the output of the system (heat) has been fed back to the input. Dr. Mohammad H. Salah Page no. 3
  • 5. Control Systems The control system is now a Closed Loop system. Information from the output goes back to the input in a Feedback loop. The comparison block of the system is normally represented by the special symbol: This shows the place of the heater thermostat in the control system. It compares the set temperature with the actual temperature. Control Systems A difference between these two temperatures is an error. When the control system detects an error, it tries to make it smaller by changing the output. This system now has all the basic elements of any control system: A demand - this is the set temperature shown above. A sensor to measure the output - a temperature sensor. This is part of the thermostat A controller - the thermostat An actuator - the heater Dr. Mohammad H. Salah Page no. 4
  • 6. Control Systems Remember: A sensor is a device that converts a physical signal (such as heat, light, sound or movement) into an electrical signal. An actuator is a device that converts an electrical signal into a physical signal (such as heat, light, sound or movement). Actuators and sensors are both devices that change one kind of signal into a different kind of signal. Control Systems The controller should be designed with some objective in mind. Typical objectives are: fastest response - reach the setpoint as fast as possible (e.g., hard drive speed) smooth response - reduce acceleration and jerks (e.g., elevators) energy efficient - minimize energy usage (e.g., industrial oven) noise immunity - ignores disturbances in the system (e.g., variable wind gusts) Dr. Mohammad H. Salah Page no. 5
  • 7. Continuous Control Systems Continuous processes require continuous sensors and/or actuators. In continuous control systems the inputs are sending information into the system all the time and the outputs of the system are being controlled all the time. A change to the input leads directly to a change in the output. An example of this kind of system is a security floodlight that comes on in the dark; the level of light reaching the light sensor is continually controlling whether or not the lamp is on. Continuous Control Systems Another example is filling a washing machine with water uses a continuous control system that monitors the water level and controls the water input valves. Continuous control systems typically need a target value, this is called a setpoint Dr. Mohammad H. Salah Page no. 6
  • 8. Continuous Control Systems Water Tank Level Control Sequential Control Systems In a sequential control system a series of different events takes place one after the other. The finishing of one event in the sequence provides the signal for the next event to start. Examples of sequential systems are: the timers that control central heating systems. washing machines. traffic lights. lifts in buildings. Dr. Mohammad H. Salah Page no. 7
  • 9. Sequential Control Systems Sometimes one of the events in the sequence is itself a continuous control system. However this is only one event in the series of events that makes up the complete sequential control system for the washing machine Synchronous Control In a fully synchronous control system all of the events in the sequence take place at set points in time, regardless of any external change. Synchronous control systems are used where the control of a sequence of events must take place at pre-set time intervals. Such a system doesn’t take any account of events outside it, only the time between events is important. Therefore it doesn’t need any sensors; it is an open loop control system. Central heating timers are synchronous controllers; the points at which the heating and hot water systems are turned on and off are fixed in time. Dr. Mohammad H. Salah Page no. 8
  • 10. Synchronous Control Once the heating or hot water is turned on, that part of the sequence is usually a continuous system; temperature is continuously monitored to control the heating system. Asynchronous Control In an asynchronous control system all of the events in the sequence take place as a result either due to an external event or because the previous event has finished, regardless of the time taken. Asynchronous control systems are used where the time taken for a sequence to occur is unimportant. Each event happens as soon as the previous event is finished or when something outside the system happens; such systems require sensors to detect the completion of an event or an outside event and so must be closed loop. Dr. Mohammad H. Salah Page no. 9
  • 11. Asynchronous Control The control system for a lift is asynchronous; the sequence of events depends entirely on external events (people pressing the call buttons outside the lift, and the floor buttons inside it) or the completion of lift movements (the lift stops moving, and the doors are opened, when a switch detects that a floor has been reached). Mixed Syn./Asyn. Control Systems In most real sequential control systems there is a mixture of synchronous and asynchronous control Many modern traffic light sets have pedestrian crossing lights or sensors in the road to detect the presence of cars. These affect the timing of the sequence of lights making the mainly synchronous system mixed. In a lift automatic doors often stay open for a fixed time. This makes a mainly asynchronous system mixed. The following is a list of some devices that use sequential control systems: 1. A security gate. 2. A dishwasher 3. A time lock on a bank’s safe. 4. A robot arm welding parts of a car together Dr. Mohammad H. Salah Page no. 10
  • 12. Mixed Syn./Asyn. Control Systems For each system; Draw a block diagram showing the sequence of events in the system. Write down whether you think it is synchronous, asynchronous or mixed. Explain your answer. If you think a system is asynchronous, explain how you think each step in the sequence is triggered. If you think it is a mixed system, describe which parts you think are synchronous and which asynchronous. Implementation of Syn. Control Sys. The heart of a synchronous control system is some kind of timer. This can be mechanical or electronic. The timer also needs: A sequencing element; this sets the times that outputs are switched on and off. Remember that there are no external inputs into a synchronous timer. An output stage that provides the start and stop signals. Dr. Mohammad H. Salah Page no. 11
  • 13. Implementation of Syn. Control Sys. Mechanical Systems All mechanical timers are different kinds of cam timer. Here a motor turning at a constant speed is used to turn lots of cams. As the cams turn they push on switches to turn them on or off. Central heating and washing machine timers always used to be made from cam timers. In industry too, cam timers have been used widely - though they are being rapidly replaced by electronic timers these days. Implementation of Syn. Control Sys. Electronic Systems There are a number of different electronic systems that can be used. A dedicated circuit uses an oscillator to give electronic clock pulses. Further circuitry, often involving the use of logic gates, is then used to control a sequence of switching. Programmable Logic Controllers (PLCs) are commonly used in industry. A PLC contains the same kind of microprocessor as a computer. However it is designed to be used in an industrial setting so is very robust. Dr. Mohammad H. Salah Page no. 12
  • 14. Implementation of Syn. Control Sys. Electronic Systems The timing sequence can be programmed either through a computer or with a small, hand held, programmer. PLCs are replacing cam timers in most places in industry. Implementation of Syn. Control Sys. Electronic Systems An important thing to note about these electronic systems is that they all use low voltages and currents. They also need to be able switch powerful outputs. So they will need some kind of output interface that protects the circuit and provides power. Relays are very often used for this. Dr. Mohammad H. Salah Page no. 13
  • 15. Relay Control Systems The relay based systems are control systems that use relays or contactors or both to operate the system actuators sequentially and they are usually electromechanical devices (some are solid state relays). Relay Control Systems Contactor can handle higher load currents than relays. The behavior of a relay or a contactor (electromechanical devices) exhibits nonlinearity in operation Dr. Mohammad H. Salah Page no. 14
  • 16. Relay Control Sys. - Features Group of relays with large number of contacts Space required Fixed application Simple control tasks Difficult expansion and/ or modification Slow action (except for solid state relays) BUT Relays continue to be used as output devices being ideal for the conversion of small signals to higher current / voltage driving signal. Relay Ladder Logic Control Logic control is used with relatively simple ON/OFF systems like pneumatics Pneumatic System Relay Ladder Logic Control Dr. Mohammad H. Salah Page no. 15
  • 17. Relay Ladder Logic Control Relay Ladder Logic Control Dr. Mohammad H. Salah Page no. 16
  • 18. Relay Ladder Logic Control Normally Open Schematic Normally Closed Schematic Relay Ladder Logic Control Output Schematic Dr. Mohammad H. Salah Page no. 17
  • 19. Relay Ladder Logic Control Why is it called “Logic Control”? Relay Ladder Logic Control Write the logic for this rung. Dr. Mohammad H. Salah Page no. 18
  • 20. Relay Ladder Logic Control Relay Ladder Logic Control Dr. Mohammad H. Salah Page no. 19
  • 21. Relay Ladder Logic Control Relay Ladder Logic Control Dr. Mohammad H. Salah Page no. 20
  • 22. Relay Ladder Logic Control Relay Ladder Logic Control Dr. Mohammad H. Salah Page no. 21
  • 23. Relay Ladder Logic Control Relay Ladder Logic Control Dr. Mohammad H. Salah Page no. 22
  • 24. Relay Ladder Logic Control Relay Ladder Logic Control Dr. Mohammad H. Salah Page no. 23
  • 25. Transducer A transducer is any device that converts energy from one form to another. Amplifier Output transducer (speaker) converts electric energy to sound energy Input transducer (microphone) converts sound energy to electric energy Sensors Sensors are input transducers used for detecting and often measuring the magnitude of something. They convert mechanical, magnetic, thermal, optical, and chemical variations into electric voltages and currents. Photoelectric sensor Dr. Mohammad H. Salah Page no. 24
  • 26. Sensors Sensors provide the equivalent of eyes, ears, nose, and tongue to the microprocessor brain. Microprocessor Optical sensor Gas sensor Microphone Probe Proximity Sensor Proximity sensors or switches detect the presence of an object without making physical contact with it. Dr. Mohammad H. Salah Page no. 25
  • 27. Proximity Sensor Applications The object being detected is too small, lightweight, or soft to operate a mechanical switch. Rapid response and high switching rates are required. An object has to be sensed through nonmetallic barriers such as glass, plastic, and paper cartons. Hostile environments conditions exist. Long life and reliable service are required. A fast electronic control system requires a bounce-free input signal. Inductive Proximity Sensor Operation Barrel type Block diagram As the target moves into the sensing area, the sensor switches the output ON Dr. Mohammad H. Salah Page no. 26
  • 28. Proximity Sensor Connections The method of connecting and exciting a proximity sensor varies with the type of sensor and its application. Target L1 L2 Load Two-wire sensor connection Proximity Sensor Connections Current-sourcing output (PNP) Sensor Control output Load Dr. Mohammad H. Salah Load is connected between the sensor and ground Page no. 27
  • 29. Proximity Sensor Connections Sensor Current-sinking output (NPN) Load Load is connected between the positive supply and sensor Control output Proximity Sensor Connection To Input Module A proximity sensor should L1 be powered continuously L2 Input module Proximity sensor The use of a bleeder resistor allows enough current for the sensor to operate but not enough to turn on the input of the PLC Bleeder resistor Dr. Mohammad H. Salah Page no. 28
  • 30. Capacitive Proximity Sensor A capacitive proximity sensor can be actuated by both conductive and nonconductive material such as wood, plastics, liquids, sugar flour and wheat. Operation is similar to that of inductive proximity sensor. Instead of a coil, the active face of the sensor is formed by two metallic electrodes – rather like an "opened capacitor". Magnetic Switch (Reed Switch) A magnetic switch (also called a reed switch) is composed of flat contact tabs that are hermetically sealed (air-tight). Magnet N S NO Common Dr. Mohammad H. Salah NC The switch is actuated by a magnet. Page no. 29
  • 31. Reed Switch Activation Reed switch Proximity motion – movement of the switch or magnet will activate the switch Magnet Rotary motion – switch is actuated twice for every complete revolution Shielding – the shield short circuits the magnetic field; switch is activated by removal of the shield Photovoltaic Or Solar Cell The photovoltaic cell, or solar cell, is a common light-sensor device that converts light energy directly into electric energy. Solar cell The solar cell converts light impulses directly into electrical charges which can easily be amplified to provide an input signal to a PLC. Dr. Mohammad H. Salah Page no. 30
  • 32. Photoconductive Or Photoresistive Cell The photoconductive cell, or photoresistive cell, is is another popular type of light transducer. Light energy falling on this device will cause a change in the resistance of the cell. Ohms 20 Ohms Light resistance 5,000 Ohms Dark resistance Photoelectric Sensor Operation Most industrial photoelectric sensors use a light-emitting diode (LED) for the light source and a phototransistor to sense the presence or absence of light. Light detector Object to be sensed Light source Dr. Mohammad H. Salah Light from the LED falls on the input of the phototransistor and the amount of conduction through the transistor changes. Analog outputs provide an output proportional to the quantity of light seen by the photodetector. Page no. 31
  • 33. Reflective Photoelectric Sensor Emits a light beam (visible, infrared, or laser) from its light emitting element and detects the light being reflected. Diffused-reflective type Retro-reflective type Emitter/receiver Operating Operating range range Target Reflector Through-Beam Type Photoelectric Sensor A through-beam photoelectric sensor is used to measure the change in light quantity caused by the target's crossing the optical axis. Emitter Operating range Receiver Target Dr. Mohammad H. Salah Page no. 32
  • 34. Bar Code Systems Bar code systems can be used to enter data much more quickly than manual methods, and are highly accurate. Diverter Decoder Scanner The decoder receives the signal from the scanner and converts these data into the character data representation of the symbol's code. PLC Ultrasonic Sensor An ultrasonic sensor operates by sending sound waves towards the target and measuring the time it takes for the pulses to bounce back. The returning echo signal is electronically converted to a 4 mA to 20 mA output, which supplies flow rate to external control devices. Dr. Mohammad H. Salah Page no. 33
  • 35. Strain/Weight Sensors A strain gauge transducer converts a mechanical strain into an electric signal. Hopper Wire type Force The load cell provides sensor input to the controller, which displays the weight and controls the hopper chute. Controller The force applied to the gauge causes the gauge to bend. This bending action also ON/OFF Chute distorts the physical size of the gauge, Control which in turn changes its resistance. Load cell Temperature Sensors Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor, and IC sensor. The thermocouple consists of a pair of dissimilar conductors fused together at one end to form the "hot" or measuring junction, with the free ends available for connection to the "cold" reference junction. A temperature difference between the measuring and reference junction generates a small DC signal voltage. Dr. Mohammad H. Salah Page no. 34
  • 36. Temperature Sensors Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor, and IC sensor. The resistance temperature detector (RTD) varies in resistance value with changes in temperature. RTD Temperature Sensors Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor, and IC sensor. The thermistor varies in resistance value with changes in temperature Dr. Mohammad H. Salah Page no. 35
  • 37. Temperature Sensors Temperature sensors convert heat into an electric signal. There are four basic types used: thermocouple, resistance temperature detector (RTD), thermistor, and IC sensor. The Integrated Circuit (IC) temperature sensor produces changes in voltage or current with changes in temperature. Flow Measurement The usual approach used in measuring fluid flow is to convert the kinetic energy that the fluid has into some other measurable form. Turbine Flow Meter Coil Flow Dr. Mohammad H. Salah Magnet Turbine The turbine blades turn at a rate proportional to the fluid velocity and are magnetized to induce voltage pulses coil. Page no. 36
  • 38. Flow Measurement The usual approach used in measuring fluid flow is to convert the kinetic energy that the fluid has into some other measurable form. Electronic Magnetic Flow Meter Can be used with electrically conducting fluids and offers no restriction to flow. A coil in the unit sets up a magnetic field. If a conductive liquid flows through this magnetic field, a voltage is induced and sensed by two electrodes. Velocity/RPM Sensors A tachometer is a small permanent magnet DC generator which when rotated produces a voltage that is directly proportional to the speed at which it is driven. Controller M Motor Tach Dr. Mohammad H. Salah Tachometers coupled are to motors commonly used in motor speed control applications to provide a feedback voltage to the controller that is proportional to motor speed. Page no. 37
  • 39. Velocity/RPM Sensors The rotating speed of a shaft is often measured using a magnetic (inductive) pickup sensor. Pickup coil Sensor output Pole piece N S Magnet A magnet is attached to the shaft. A small coil of wire held near the magnet receives a pulse each time the magnet passes. By measuring the frequency of the pulses, the shaft speed can be determined. 0V Output Control Devices A variety of output control devices can be operated by the controller output module to control traditional processes. These include: Pilot light Heater Dr. Mohammad H. Salah Control relay Solenoid Motor starter Alarm Solenoid valve Small motor Page no. 38
  • 40. Actuators An actuator is any device that converts an electrical signal into mechanical movement. The principle types of actuators are relays, solenoids, and motors. The solenoid converts electric current into linear motion. Symbol Solenoid Plunger AIR Coil Solenoid Valve A solenoid valve is a combination of: a solenoid with its core or plunger a valve body containing an orifice in which a disc or plug is positioned to restrict or allow flow Forward motion of piston Directional solenoid valve When SOL A is energized, the valve spool is shifted to redirect the fluid and move the cylinder forward CR SOL A FWD SOL A CR Dr. Mohammad H. Salah Page no. 39
  • 41. Stepper Motor A stepper motor converts electrical pulses applied to it into discrete rotor movements called steps. They are used to provide precise position control of movement. Stepper motor control system Module Communicates with the PLC and responds with pulse trains Stepper-motor translator Enables control of the stepper motor Step motor The motor will move one step for each pulse received PLC Control of a Large Motor Load When a PLC needs to control a large motor, it must work in conjunction with a starter. Motor starters are in various available standard National Electric manufacturers (NEMA) sizes and ratings. Dr. Mohammad H. Salah Page no. 40
  • 42. Programmable Logic Controller Outlines Introduction Advantages of PLC Control Systems PLC Versus Other Types of Control Typical Areas of PLC Applications PLC Product Application Ranges Structure and Hardware PLC Scan Process PLC Programming Modes of Operation PLC and Networks Dr. Mohammad H. Salah Page no. 41
  • 43. Introduction A programmable logic controller (PLC) is a specialized computer used to control machines and process. PLC uses a programmable memory to store instructions and execute specific functions that include On/Off control, timing, counting, sequencing, arithmetic, and data handling. The word Programmable differentiates it from the conventional hard-wired relay logic. Introduction PLCs are used in both SCADA and DCS systems as the control components of an overall hierarchical system to provide local management of processes through feedback control Dr. Mohammad H. Salah Page no. 42
  • 44. Introduction Using a PLC requires setting up the hardware and software The hardware installation consists of wiring the PLC to all switches and sensors of the system and to such output devices as relay coils, indicator lamps, or small motors Introduction The control program is usually developed on a PC, using software provided by the PLC manufacturer This software allows the user to develop the control program on the monitor screen Once the program is complete, it is automatically converted into instructions for the PLC processor The completed program is then downloaded into the PLC Once the program is in the PLC’s memory, the programming terminal can be disconnected, and the PLC will continue to function on its own Dr. Mohammad H. Salah Page no. 43
  • 45. Advantages of PLC Control Sys. Eliminates much of the hard wiring that was associated with conventional relay control circuits: The PLC also surpassed the hazard of changing the wiring. The program takes the place of the external wiring that would be required to control the process Advantages of PLC Control Sys. Increased Reliability: Once a program has been written and tested, it can be downloaded to other PLCs. Since all the logic is contained in the PLC’s memory, there is no chance of making a logic wiring error. Dr. Mohammad H. Salah Page no. 44
  • 46. Advantages of PLC Control Sys. Faster Response Time: PLCs operate in real-time which means that an event taking place in the field will result in an operation or output taking place. Machines that thousands of process items per second and objects that spend only a fraction of a second in front of a sensor require the PLC’s quick response capability. Advantages of PLC Control Sys. More Flexibility: Original equipment manufacturers (OEMs) can provide system updates for a process by simply sending out a new program. It is easier to create and change a program in a PLC than to wire and rewire a circuit. End-users can modify the program in the field. Dr. Mohammad H. Salah Page no. 45
  • 47. Advantages of PLC Control Sys. Lower Cost: Originally PLCs were designed to replace relay logic control. The cost savings using PLCs have been so significant that relay control is becoming obsolete, except for power applications. Generally, if an application requires more than about 6 control relays, it will usually be less expensive to install a PLC. Advantages of PLC Control Sys. Communication Capabilities: PLC can communicate with other controllers or computer equipment. They can be networked to perform such functions as: supervisory control, data gathering, monitoring devices and process parameters, and downloading and uploading of programs. Dr. Mohammad H. Salah Page no. 46
  • 48. Advantages of PLC Control Sys. Easier to Troubleshoot: PLCs have resident diagnostic and override functions that allows users to easily trace and correct software and hardware problems. The control program can be watched in real-time as it executes to find and fix problems Advantages of PLC Control Sys. PLCs can work with the help of the HMI (Human-Machine Interface) computer HMI Dr. Mohammad H. Salah Page no. 47
  • 49. PLC Versus Other Types of Control PLCs Versus Relay Control Today’s demand for high quality and productivity can hardly be fulfilled economically without electronic control equipment. With rapid technology developments and increasing competition, the cost of programmable controls has been driven down to the point where a PLC-versus-relay cost study is no longer necessary or valid. When deciding whether to use a PLC-based system or a hardwired relay system, the designer must ask several questions. Some of these questions are: PLC Versus Other Types of Control PLCs Versus Relay Control Is there a need for flexibility in control logic changes? Is there a need for high reliability? Are space requirements important? Are increased capability and output required? Are there data collection requirements? Will there be frequent control logic changes? Will there be a need for rapid modification? Must similar control logic be used on different machines? Is there a need for future growth? What are the overall costs? Dr. Mohammad H. Salah Page no. 48
  • 50. PLC Versus Other Types of Control PLCs Versus Relay Control Even in a case where no flexibility or future expansion is required, a large system can benefit tremendously from the troubleshooting and maintenance aids provided by a PLC. The extremely short cycle (scan) time of a PLC allows the productivity of machines that were previously under electromechanical control to increase considerably. Also, although relay control may cost less initially, this advantage is lost if production downtime due to failures is high. PLC Versus Other Types of Control PLCs Versus Relay Control Dr. Mohammad H. Salah Page no. 49
  • 51. PLC Versus Other Types of Control PLCs Versus Computer Control Unlike computers, PLCs are specifically designed to survive the harsh conditions of the industrial environment. A well-designed PLC can be placed in an area with substantial amounts of electrical noise, electromagnetic interference, mechanical vibration, and non-condensing humidity. PLC’s hardware and software are designed for easy use by plant electricians and technicians. the software programming uses conventional relay ladder symbols, or other easily learned languages, which are familiar to plant personnel. PLC Versus Other Types of Control Dr. Mohammad H. Salah Page no. 50
  • 52. Typical Areas of PLC Applications PLC Product Application Ranges The PLC market can be segmented into five groups: 1. Micro PLCs 2. Small PLCs 3. Medium PLCs 4. Large PLCs 5. Very large PLCs The A, B, and C areas overlapping reflect enhancements, by adding options, of the standard features of the PLCs within a particular segment. Dr. Mohammad H. Salah Page no. 51
  • 53. PLC Control of a Large Motor Load When a PLC needs to control a large motor, it must work in conjunction with a starter. Motor starters are available in various standard National Electric manufacturers (NEMA) sizes and ratings. Structure and Hardware Power Supply Processor (CPU) Memories Input/output modules Programming Port PLC Bus Expansion Models Dr. Mohammad H. Salah Page no. 52
  • 54. Structure and Hardware The PLC bus are the wires which contains the data bus, address bus, and control signals. The processor uses the bus to communicate with the modules Structure and Hardware Dr. Mohammad H. Salah Page no. 53
  • 55. Structure and Hardware Power Supply PLCs are usually powered directly from 120 or 240Vac The power supply converts the AC into DC voltages for the internal microprocessor components It may also provide the user with a source of reduced voltage to drive switches, small relays, indicator lamps, and the like Structure and Hardware Processor (CPU) The processor is the brain of the PLC The processor is a microprocessor-based CPU and is the part of the PLC that is capable of reading and executing the program instructions, oneby-one (such as the rungs of a ladder logic program) Dr. Mohammad H. Salah Processor Module Page no. 54
  • 56. Structure and Hardware Processor (CPU) A special program called the operating system controls the actions of the CPU and consequently the execution of the user’s program The operating system is supplied by the PLC manufacturer and is permanently held in memory. A PLC operating system is designed to scan image memory and the main memory which stores the ladder diagram program Structure and Hardware Memories The program memory receives and holds the downloaded program instructions from the programming device This memory is usually an EEPROM (electrically erasable programmable ROM) or a battery-backup RAM, both of which are capable of retaining data Data memory is RAM memory used as a “scratch pad” by the processor to temporarily store internal and external program-generated data For example, it would store the present status of all switches connected to the input terminals and the value of internal counters and timers. Dr. Mohammad H. Salah Page no. 55
  • 57. Structure and Hardware Memories Structure and Hardware Input/Output Modules The I/O modules are interfaces to the outside world These control ports may be built into the PLC unit or, more typically, are packaged as separate plug-in modules, where each module contains a set of ports The most common type of I/O is called discrete I/O and deals with on-off devices Analog I/O modules allow the PLC to handle analog signals Dr. Mohammad H. Salah Page no. 56
  • 58. Structure and Hardware Input/Output Modules Fixed I/O configuration Is typical of small PLCs Comes in one package, with no separate removable units. The processor and I/O are packaged together. Lower in cost – but lacks flexibility. Structure and Hardware Input/Output Modules Modular I/O configuration When a module slides into the rack, it makes an electrical connection with a series of contacts called the “backplane”. The backplane is located at the rear of the rack. Dr. Mohammad H. Salah Page no. 57
  • 59. Structure and Hardware Discrete Input Modules (DIM) DIM connect real-world switches to the PLC and are available for either AC or DC voltages (typically, 240 Vac, 120 Vac, 24 Vdc, and 5 Vdc) circuitry within the module converts the switched voltage into a logic voltage for the processor Structure and Hardware Discrete Output Modules (DOM) DOM provide on-off signals to drive lamps, relays, small motors, motor starters, and other devices Several types of output ports are available: Triac outputs control AC devices, transistor switches control DC devices, and relays control AC or DC devices (and provide isolation as well) Dr. Mohammad H. Salah Page no. 58
  • 60. Structure and Hardware Analog Input Modules (AIM) An analog input module has one or more ADCs (analog-todigital converters), allowing analog sensors, such as temperature, to be connected directly to the PLC Depending on the module, the analog voltage or current is converted into an 8-, 12-, or 16-bit digital word Structure and Hardware Analog Output Modules (AOM) An analog output module contains one or more DACs (digital-to-analog converters), allowing the PLC to provide an analog output—for example, to drive a DC motor at various voltage levels Dr. Mohammad H. Salah Page no. 59
  • 61. Structure and Hardware Input/Output Modules Specialized modules that perform particular functions are available for many PLCs. Examples include: Thermocouple module — Interfaces a thermocouple to the PLC. Motion-control module — Runs independently to control muti-axis motion in a device such as a robot Communication module — Connects the PLC to a network High-speed counter module — Counts the number of input pulses for a fixed period of time PID module — An independently running PID selfcontained controller (PID control can also be implemented with software, as described later in this chapter) Structure and Hardware Input/Output Modules Dr. Mohammad H. Salah Page no. 60
  • 62. Structure and Hardware Interpreting I/O Specificaions Electrical: I/O Voltage Rating. I/O Current Rating. Input Threshold Voltage. Input Delay. Off-State Leakage Current. Output Power Rating. Surge Current (Max). Output On-Delay. Output Off-Delay. Digital Resolution. Structure and Hardware Interpreting I/O Specificaions Mechanical: Points Per Module. Wire Size. Environmental: Ambient Temperature Rating. Humidity. Dr. Mohammad H. Salah Page no. 61
  • 63. Structure and Hardware Programming Port and Device The programming port receives the downloaded program from the programming device (usually a PC) Structure and Hardware Programming Port and Device The PLC does not have a front panel or a monitor; thus, to “see” what the PLC is doing (for debugging or troubleshooting), you must connect it to a PC Dr. Mohammad H. Salah Page no. 62
  • 64. Structure and Hardware Programming Port and Device A personal computer (PC) is the most commonly used programming device. The computer monitor is used to display the logic on the screen. The personal computer communicates with the PLC processor via a serial or parallel data communications link. The software allows users to create, edit, document, store and troubleshoot programs. If the programming unit is not in use, it may be unplugged and removed. Removing the programming unit will not affect the operation of the user program. Structure and Hardware Programming Port and Device Hand-held programming devices are sometimes used to program small PLCs. They are compact, inexpensive, and easy to use, but are not able to display as much logic on screen as a computer monitor. Hand-held units are often used on the factory floor for troubleshooting, modifying programs, and transferring programs to multiple machines. Dr. Mohammad H. Salah Page no. 63
  • 65. Structure and Hardware Expansion Modules Most PLCs are expandable Expansion modules contain additional inputs and outputs These are connected to the base unit using a ribbon connector BIG PICTURE Dr. Mohammad H. Salah Page no. 64
  • 66. PLC Scan Process PLC Scan Process Dr. Mohammad H. Salah Page no. 65
  • 67. PLC Scan Process PLC Scan Process The scan time is dependent on the clock frequency of the processor. Misunderstanding the way the PLC scans can cause programming bugs! Dr. Mohammad H. Salah Page no. 66
  • 68. PLC Scan Process PLC Scan Process Data Flow Overview Dr. Mohammad H. Salah Page no. 67
  • 69. PLC Scan Process PLC Programming The term PLC programming language refers to the method by which the user communicates information to the PLC. A PLC program is not actually a wiring diagram but a way to describe the logical relationship between inputs and outputs The PLC programming languages are: – Sequential Control and State Graph (Graph) – Sequential Function Chart (SFC) – Structured Text (ST) – Instruction List (IL) – Function Block Diagram (FBD) – Ladder Diagram (LD) The most common is LD, FBD, and IL but the most use is the LD. Dr. Mohammad H. Salah Page no. 68
  • 70. PLC Programming Sequential Functional Chart Sequential functional chart, or SFC, is a graphical “language” that provides a diagrammatic representation of control sequences in a program. Basically, sequential function chart is a flowchart-like framework that can organize the subprograms or subroutines (programmed in LD, FBD, IL, and/or ST) that form the control program. SFC is particularly useful for sequential control operations, where a program flows from one step to another once a condition has been satisfied (TRUE or FALSE). The SFC programming framework contains three main elements that organize the control program: steps, transitions, and actions. PLC Programming Sequential Functional Chart Dr. Mohammad H. Salah Page no. 69
  • 71. PLC Programming Sequential Functional Chart PLC Programming Structured Text Dr. Mohammad H. Salah Page no. 70
  • 72. PLC Programming Instruction List PLC Programming Function Block Diagram Dr. Mohammad H. Salah Page no. 71
  • 73. PLC Programming Ladder Diagram PLC Programming Ladder Diagram A LAD (special kind of wiring diagram) was developed to document electromechanical control circuits. Ladder diagram programs are highly symbolic and are the result of years of evolution of industrial control circuit diagrams This type of diagram has two vertical wires (rails) on either side of the drawing to supply the power Each rung of the ladder diagram connects from one rail to the other and is a separate circuit, which typically consists of some combination of switches, relay contacts, relay coils, and motors It is common for the coil of a relay to be in one rung and the contacts to be in another Dr. Mohammad H. Salah Page no. 72
  • 74. PLC Programming Ladder Diagram PLC Programming Ladder Diagram Ladder rung Ladder rail Control scheme is drawn between two vertical supply lines. Dr. Mohammad H. Salah Page no. 73
  • 75. PLC Programming Ladder Diagram - Comparison PLC Programming Ladder Diagram - Comparison Dr. Mohammad H. Salah Page no. 74
  • 76. PLC Programming Relay-Type Instructions PLC Programming Examine if Closed (XIC) Instruction Dr. Mohammad H. Salah Page no. 75
  • 77. PLC Programming Examine if Closed (XIC) Instruction PLC Programming Examine if Closed (XIC) Instruction Dr. Mohammad H. Salah Page no. 76
  • 78. PLC Programming Examine if Closed (XIC) Instruction PLC Programming Examine if Open (XIO) Instruction Dr. Mohammad H. Salah Page no. 77
  • 79. PLC Programming Examine if Open (XIO) Instruction PLC Programming Examine if Open (XIO) Instruction Dr. Mohammad H. Salah Page no. 78
  • 80. PLC Programming Output Energize (OTE) Instruction PLC Programming Output Energize (OTE) Instruction Dr. Mohammad H. Salah Page no. 79
  • 81. PLC Programming Output Energize (OTE) Instruction PLC Programming Status Bit Example Dr. Mohammad H. Salah Page no. 80
  • 82. PLC Programming Status Bit Example PLC Programming Ladder Rung Dr. Mohammad H. Salah Page no. 81
  • 83. PLC Programming Rung Continuity PLC Programming Rung Continuity Dr. Mohammad H. Salah Page no. 82
  • 84. PLC Programming Example PLC Programming Parallel Input Branch Instruction Dr. Mohammad H. Salah Page no. 83
  • 85. PLC Programming Parallel Output Branching PLC Programming Nested Input and Output Branches Dr. Mohammad H. Salah Page no. 84
  • 86. PLC Programming Nested Contact Program PLC Programming PLC Matrix Limitation Diagram Dr. Mohammad H. Salah Page no. 85
  • 87. PLC Programming Programming of Vertical Contacts PLC Programming Programming for Different Scan Patterns Dr. Mohammad H. Salah Page no. 86
  • 88. PLC Programming Internal (Auxiliary) Control Relay PLC Programming Internal (Auxiliary) Control Relay Dr. Mohammad H. Salah Page no. 87
  • 89. PLC Programming Operation of XIC/XIO Instruction PLC Programming Operation of XIC/XIO Instruction Dr. Mohammad H. Salah Page no. 88
  • 90. PLC Programming Ladder Diagram - Timers The Timer instruction provides a time delay, performing the function of a time-delay relay (e.g., controlling the time for a mixing operation or the duration of a warning beep) The length of time delay is determined by specifying a preset value The timer is enabled when the rung conditions become TRUE Once enabled, it automatically counts up until it reaches the Preset value and then goes TRUE (and stays TRUE) There are two types of time delay (On and Off) PLC Programming Ladder Diagram - Timers Dr. Mohammad H. Salah Page no. 89
  • 91. PLC Programming Ladder Diagram - Timers On-Delay Timer Off-Delay Timer PLC Programming Ladder Diagram - Timers Dr. Mohammad H. Salah Page no. 90
  • 92. PLC Programming Ladder Diagram - Timers PLC Programming Ladder Diagram - Timers Dr. Mohammad H. Salah Page no. 91
  • 93. PLC Programming Ladder Diagram - Timers PLC Programming Ladder Diagram - Counters A Counter instruction keeps track of the number of times some event occurs (e.g., the count could represent the number of parts to be loaded into a box) Counters may be either count-up or count-down types. The Counter will increment (or decrement) every time the rung makes a FALSE-to-TRUE transition The count is retained until a RESET instruction (with the same address as the Counter) is enabled The Counter has a Preset value associated with it. When the count gets up to the Preset value, the output goes TRUE. This allows the program to initiate some action based on a certain count Dr. Mohammad H. Salah Page no. 92
  • 94. PLC Programming Ladder Diagram - Counters PLC Programming Ladder Diagram - Counters Dr. Mohammad H. Salah Page no. 93
  • 95. PLC Programming Ladder Diagram - Counters PLC Programming Ladder Diagram - Counters Dr. Mohammad H. Salah Page no. 94
  • 96. PLC Programming Ladder Diagram - Sequencers The Sequencer instruction is used when a repeating sequence of outputs is required Traditionally, electromechanical sequencers (Figure 12.10) were used in this type of application (where a drum rotates slowly, and cams on the drum activate switches) The Sequencer instruction allows the PLC to implement this common control strategy PLC Programming Ladder Diagram - Sequencers Dr. Mohammad H. Salah Page no. 95
  • 97. PLC Programming Ladder Diagram - Sequencers PLC Programming Ladder Diagram - Sequencers Dr. Mohammad H. Salah Page no. 96
  • 98. PLC Programming Ladder Diagram - Comparators The temperature in an electric oven is to be maintained by a 16-bit PLC at approximately 100°C, using twopoint control (actual range: 98102°). An oven with an electric heating element driven by a contactor (high-current relay), an LM35 temperature sensor (produces 10 mV/°C), an operator on-off switch, and the PLC. The PLC has a processor and three I/O modules: a discrete input module (slot 1), a 16bit analog input module (slot 2), and a discrete output module (slot 3). Draw the ladder diagram for this system PLC Programming Ladder Diagram - Comparators Dr. Mohammad H. Salah Page no. 97
  • 99. PLC Programming Equivalent Ladder / Logic Symbols PLC Programming Equivalent Ladder / Logic Symbols Dr. Mohammad H. Salah Page no. 98
  • 100. PLC Programming Equivalent Ladder / Logic Symbols PLC Programming Equivalent Ladder / Logic Symbols Dr. Mohammad H. Salah Page no. 99
  • 101. PLC Programming Equivalent Ladder / Logic Symbols PLC Programming Equivalent Ladder / Logic Symbols Dr. Mohammad H. Salah Page no. 100
  • 102. PLC Programming Equivalent Ladder / Logic Symbols PLC Programming Equivalent Ladder / Logic Symbols Dr. Mohammad H. Salah Page no. 101
  • 103. PLC Programming Equivalent Ladder / Logic Symbols PLC Programming Equivalent Ladder / Logic Symbols Dr. Mohammad H. Salah Page no. 102
  • 104. PLC Programming Equivalent Ladder / Logic Symbols PLC Programming Ladder Diagram – Programming Comments Arranging Instructions for Optimum Performance There is more than one way to correctly implement the ladder logic. In some cases one arrangement may be more efficient in terms of the amount of memory used and the time required to scan the program. Sequence series instructions from the most likely to be FALSE (far left) to least likely to be FALSE (far right) Instruction LEAST Instruction MOST likely to be FALSE likely to be FALSE Once a processor sees a FALSE input instruction in series, it executes the remaining instructions FALSE, even if they are TRUE Dr. Mohammad H. Salah Page no. 103
  • 105. PLC Programming Ladder Diagram – Programming Comments Arranging Instructions for Optimum Performance If your rung contains parallel branches, place the path that is most often TRUE on the top. The processor will not look at the others unless the top path is FALSE. Path most likely to be TRUE LESS likely LEAST likely Modes of Operation Dr. Mohammad H. Salah Page no. 104
  • 106. Modes of Operation PLC and Networks Physically, a network is a wire acting as an “electronic highway” that can pass messages between nodes (PCs and other electronic devices) Each node on the network has a unique address, and each message called a data packet (includes the address of where it’s going and where it came from) All data on the network is sent serially (one bit at a time) on one wire The most common type of network uses the bus topology, which means that all the nodes tap into a single cable Dr. Mohammad H. Salah Page no. 105
  • 107. PLC and Networks PLC and Networks There are three good reasons for using a network: A device network simplifies wiring. Clearly the network is a simpler system that uses less wire. This reduces the amount of wiring needed With a network, the sensor data arrives in better shape. In the traditional system, a low-level analog voltage may have to travel many feet. The signal is subject to attenuation and noise and other losses Network devices tend to be more intelligent. For example, a photo cell could send a message saying the light level has diminished, (indicating that the lens may be getting dirty or that someone has bumped it out of position) Dr. Mohammad H. Salah Page no. 106
  • 108. PLC and Networks PLC and Networks Three levels of networks Dr. Mohammad H. Salah Page no. 107
  • 109. PLC and Networks Dr. Mohammad H. Salah Page no. 108
  • 110. PLC Exercises Ladder Diagram Programming Steps for Building a Ladder Diagram 1. 2. 3. 4. 5. 6. 7. 8. Determine the No. of digital I/O Determine the No. of analog I/O (if needed) Determine if there are special functions in the process Estimate program capacity depending on the process Choose a suitable PLC series Prepare the wiring diagram Draw flowchart or control diagram (Optional) Program the PLC using the ladder diagram Dr. Mohammad H. Salah Page no. 109
  • 111. Exercise #1: Moving a Pneumatic Piston Control Problem The PLC task is to move the piston in and out. When switch SW1 is momentarily turned on, piston A is to move out of the cylinder in A+ direction. When switch SW2 is momentarily turned on, piston A is to move into the cylinder in A- direction. Exercise #1: Moving a Pneumatic Piston Dr. Mohammad H. Salah Page no. 110
  • 112. Exercise #1: Moving a Pneumatic Piston If SW1 and SW2 are pressed together, what would happen? The two solenoid valves will be tuned off How can we make an electrical interlock? Use the contacts of the main relays instead of the input contacts Exercise #2: Sequencing of Pneumatic Pistons Control Problem The PLC task is to operate piston A followed by piston B followed by piston C. The sequence is A+, A-, B+, B-, C+, C- is to be repeated when switch SW1 is turned on Dr. Mohammad H. Salah Page no. 111
  • 113. Exercise #2: Sequencing of Pneumatic Pistons Exercise #2: Sequencing of Pneumatic Pistons If the system does not work or sequence in not correct, what would be the possible reasons? • Solenoid valves do not work • The wiring of solenoid valves is not correct or not in the correct order (wiring problem) • The ladder diagram is not properly written (sequence in not correct) Dr. Mohammad H. Salah Page no. 112
  • 114. Exercise #3: Batching Machine Control Problem The PLC task is to control a simple machine which counts and batches components moving along a conveyor. It is required that ten components be channeled down route A and twenty components down route B. A reset facility is required Exercise #3: Batching Machine Dr. Mohammad H. Salah Page no. 113
  • 115. Exercise #3: Batching Machine Exercise #3: Batching Machine If the system does not batch and/or count, what would be the possible reasons? • • • • The reset switch is always on The microswitch does not work The flap solenoid does not work The ladder diagram is not properly written Dr. Mohammad H. Salah Page no. 114
  • 116. Exercise #4: Reject Machine Control Problem The PLC task is to detect and reject faulty components. Components are transported on a conveyor past a retro-reflective type photoelectric switch. The photoelectric switch is positioned at a height (H) above the conveyor where (H) represents a tolerance value for component height. Good components pass underneath the photoelectric switch and no signal is generated. Faulty components break the light beam twice as they pass the photoelectric switch. Exercise #4: Reject Machine Dr. Mohammad H. Salah Page no. 115
  • 117. Exercise #4: Reject Machine Exercise #4: Reject Machine If the system does not reject faulty components, what would be the possible reasons? • The photoelectric switch is too high (H is too big) • The photoelectric switch does not work • The pneumatic blower does not work • The ladder diagram is not properly written • The faulty components is not as described in the drawing Dr. Mohammad H. Salah Page no. 116
  • 118. Exercise #5: Pick and Place Unit Control Problem The PLC task is to: a) move the gripper to X+ position b) close the gripper so that it takes hold of a component c) rotate the gripper through 180o to the Θ+ position d) Release the component e) Rotate the gripper back to the Θ- position so that the pick and place operation may be repeated Exercise #5: Pick and Place Unit Dr. Mohammad H. Salah Page no. 117
  • 119. Exercise #5: Pick and Place Unit If the system does not work or sequence is not correct, what would be the possible reasons? • Wiring problem • Some solenoid valves do not work • Timing is not correct • The ladder diagram is not properly written (sequence in not correct) How can we get rid of the timers in the ladder diagram/program? Use position sensors for feedback but that would be expensive compared to using timers but more accurate and reliable in case the mechanical system starts to have some problems Exercise #6: Production Line Control Problem The PLC task is to organize the production process. Cans filled with fluid and capped before passing into a conveyor. The photoelectric switches P1 and P2 are used to check that each can has a cap. Photoelectric switch P3 provides a trigger for the ink jet printer which prints a batch number on each can. Photoelectric switch P4 is used to count three cans into the palletizing machine that transports three cans through a machine which heat shrinks a plastic wrapping over them. All photoelectric switches on the production line are of the retro reflective type. Dr. Mohammad H. Salah Page no. 118
  • 120. Exercise #6: Production Line Exercise #6: Production Line Dr. Mohammad H. Salah Page no. 119
  • 121. Exercise #6: Production Line If the system allows uncapped cans to pass, what would be the possible reasons? • • • • The height of the photoelectric switch needs to be readjusted The photoelectric switch does not work (transmitter or receiver) The photoelectric transmitter is not aligned with the receiver The ladder diagram is not properly written (or timer is not set properly) Exercise #7: Star-Delta Connection Dr. Mohammad H. Salah Page no. 120
  • 122. Exercise #7: Star-Delta Connection Exercise #7: Star-Delta Connection PLC system layout – Wiring diagram Dr. Mohammad H. Salah Page no. 121
  • 123. Exercise #7: Star-Delta Connection Exercise #8: Drilling Process PB1 A simple drilling operation requires the drill press to turn on only if there is a part present and the operator has one hand on each of the start switches. This precaution will ensure that the operator's hands are not in the way of the drill. PB2 Drill motor Switches Part sensor Dr. Mohammad H. Salah Page no. 122
  • 124. Exercise #8: Drilling Process A simple drilling operation requires the drill press to turn on only if there is a part present and the operator has one hand on each of the start switches. This precaution will ensure that the operator's hands are not in the way of the drill. Exercise #9: Motorized Door A motorized overhead garage door is to automatically to preset open and closed positions. Dr. Mohammad H. Salah be operated Page no. 123
  • 125. Exercise #9: Motorized Door Exercise #10: Continuous Filling Machine Continuous filling operation requires boxes moving on a conveyor to be automatically positioned and filled. Hooper Motor Level switch Run PL Solenoid PL Standby PL Full Photo switch START STOP Dr. Mohammad H. Salah Page no. 124
  • 126. Exercise #10: Continuous Filling Machine Exercise #11: Transporting Process 1 Need to do the PLC hardware layout + ladder diagram Dr. Mohammad H. Salah Page no. 125
  • 127. Exercise #11: Transporting Process 1 Exercise #12: Transporting Process 2 Need to automate the system using a PLC (hardware layout + ladder diagram) Dr. Mohammad H. Salah Page no. 126
  • 128. Industrial Control Systems Outlines Introduction ICS Operation ICS Key Components SCADA Systems DCS Systems RTU Telemetry Modems SCADA Systems Examples Dr. Mohammad H. Salah Page no. 127
  • 129. Introduction Industrial Control Systems is a general term that includes several types of control systems: Supervisory Control And Data Acquisition (SCADA) systems. Distributed Control Systems (DCS). Other control system configurations such as Programmable Logic Controllers (PLC). ICS are typically used in industries such as electrical, water and wastewater, oil and natural gas, chemical, transportation, pharmaceutical, food and beverage, and discrete manufacturing (e.g., automotive). These control systems are used for critical infrastructures that are often highly interconnected and mutually dependent systems Introduction SCADA systems are highly distributed systems used to control geographically dispersed assets, often scattered over thousands of square kilometers, where centralized data acquisition and control are critical to system operation. They are used in distribution systems such as water distribution and wastewater collection systems, oil and natural gas pipelines, electrical power grids, and railway transportation systems. A SCADA control center performs centralized monitoring and control for field sites over long-distance communications networks, including monitoring alarms and processing status data. Dr. Mohammad H. Salah Page no. 128
  • 130. Introduction Based on information received from remote stations, automated or operator-driven supervisory commands can be pushed to remote station control devices, which are often referred to as field devices. Field devices control local operations such as opening and closing valves and breakers, collecting data from sensor systems, and monitoring the local environment for alarm conditions. Introduction DCS are used to control industrial processes such as electric power generation, oil refineries, water and wastewater treatment, and chemical, food, and automotive production. DCS are integrated as a control architecture containing a supervisory level of control overseeing multiple, integrated sub-systems that are responsible for controlling the details of a localized process. Product and process control are usually achieved by deploying feedback or feedforward control loops whereby key product and/or process conditions are automatically maintained around a desired set point. Dr. Mohammad H. Salah Page no. 129
  • 131. Introduction To accomplish the desired product and/or process tolerance around a specified set point, specific PLCs are employed in the field and proportional, integral, and/or derivative settings on the PLC are tuned to provide the desired tolerance as well as the rate of self-correction during process upsets. DCS are used extensively in process-based industries. Introduction PLCs are computer-based solid-state devices that control industrial equipment and processes. While PLCs are control system components used throughout SCADA and DCS systems, they are often the primary components in smaller control system configurations used to provide operational control of discrete processes such as automobile assembly lines and power plant soot blower controls. PLCs are used extensively in almost all industrial processes. Dr. Mohammad H. Salah Page no. 130
  • 132. Introduction While control systems used in distribution and manufacturing industries are very similar in operation, they are different in some aspects. One of the primary differences is that DCS or PLC-controlled sub-systems are usually located within a more confined factory or plant-centric area, when compared to geographically dispersed SCADA field sites. DCS and PLC communications are usually performed using local area network (LAN) technologies that are typically more reliable and high speed compared to the long-distance communication systems used by SCADA systems. Introduction In fact, SCADA systems are specifically designed to handle long-distance communication challenges such as delays and data loss posed by the various communication media used. DCS and PLC systems usually employ greater degrees of closed loop control than SCADA systems because the control of industrial processes is typically more complicated than the supervisory control of distribution processes. Dr. Mohammad H. Salah Page no. 131
  • 133. Introduction SCADA systems are generally used to control dispersed assets using centralized data acquisition and supervisory control. DCS are generally used to control production systems within a local area such as a factory using supervisory and regulatory control. PLCs are generally used for discrete control for specific applications and generally provide regulatory control. Introduction ICS have unique performance and reliability requirements and often use operating systems and applications that may be considered unconventional to typical IT personnel. Furthermore, the goals of safety and efficiency sometimes conflict with security in the design and operation of control systems. ICS implementations were susceptible primarily to local threats because many of their components were in physically secured areas and the components were not connected to IT networks or systems. However, the trend toward integrating ICS systems with IT networks provides significantly less isolation for ICS from the outside world than predecessor systems, creating a greater need to secure these systems from remote, external threats. Dr. Mohammad H. Salah Page no. 132
  • 134. Introduction The increasing use of wireless networking places ICS implementations at greater risk from adversaries who are in relatively close physical proximity but do not have direct physical access to the equipment. Threats to control systems can come from numerous sources, including hostile governments, terrorist groups, disgruntled employees, malicious intruders, complexities, accidents, natural disasters as well as malicious or accidental actions by insiders. ICS security objectives typically follow the priority of availability, integrity and confidentiality, in that order. Introduction It is essential for a cross-functional cyber (internet) security team to share their varied domain knowledge and experience to evaluate and mitigate risk to the ICS. The cyber security team should consist of a member of the organization’s IT staff, control engineer, control system operator, network and system security expert, a member of the management staff, and a member of the physical security department at a minimum. For continuity and completeness, the cyber security team should consult with the control system vendor and/or system integrator as well. Dr. Mohammad H. Salah Page no. 133
  • 135. Introduction The cyber security team should report directly to site management (e.g., facility superintendent) or the company’s CIO/CSO, who in turn, accepts complete responsibility and accountability for the cyber security of the ICS. An effective cyber security program for an ICS should apply a strategy known as “defense-in-depth”, layering security mechanisms such that the impact of a failure in any one mechanism is minimized. CIO = Chief Information Officer or IT Director CSO = Chief Security Officer ICS Operation Dr. Mohammad H. Salah Page no. 134
  • 136. ICS Operation 1. Control Loop. A control loop consists of sensors for measurement, controller hardware such as PLCs, actuators such as control valves, breakers, switches and motors, and the communication of variables. Controlled variables are transmitted to the controller from the sensors. The controller interprets the signals and generates corresponding manipulated variables, based on set points, which it transmits to the actuators. Process changes from disturbances result in new sensor signals, identifying the state of the process, to again be transmitted to the controller. ICS Operation 2. Human-Machine Interface (HMI). Operators and engineers use HMIs to monitor and configure set points, control algorithms, and adjust and establish parameters in the controller. The HMI also displays process status information and historical information. 3. Remote Diagnostics and Maintenance Utilities. Diagnostics and maintenance utilities are used to prevent, identify and recover from abnormal operation or failures. A typical ICS contains a propagation of control loops, HMIs, and remote diagnostics and maintenance tools built using an array of network protocols on layered network architectures Dr. Mohammad H. Salah Page no. 135
  • 137. ICS Key Components 1. Control Components Control Server: hosts the DCS or PLC supervisory control software that is designed to communicate with lowerlevel control devices. The control server accesses subordinate control modules over an ICS network. SCADA Server or Master Terminal Unit (MTU): The SCADA Server is the device that acts as the master in a SCADA system. Remote terminal units (RTUs) and PLC devices located at remote field sites usually act as slaves. ICS Key Components 1. Control Components Remote Terminal Unit (RTU): It is also called a remote telemetry unit. It is a special purpose data acquisition and control unit designed to support SCADA remote stations. RTUs are field devices often equipped with wireless radio interfaces to support remote situations where wirebased communications are unavailable. Sometimes PLCs are implemented as field devices to serve as RTUs; in this case, the PLC is often referred to as an RTU. Dr. Mohammad H. Salah Page no. 136
  • 138. ICS Key Components 1. Control Components Programmable Logic Controller (PLC): The PLC is a small industrial computer originally designed to perform the logic functions executed by electrical hardware. Other controllers used at the field level are process controllers and RTUs; they provide the same control as PLCs but are designed for specific control applications. In SCADA environments, PLCs are often used as field devices because they are more economical, multipurpose, flexible, and configurable than special-purpose RTUs. ICS Key Components 1. Control Components Programmable Logic Controller (PLC): PLCs are used in both SCADA and DCS systems as the control components of an overall hierarchical system to provide local management of processes through feedback control. In the case of SCADA systems, they provide the same functionality of RTUs. When used in DCS, PLCs are implemented as local controllers within a supervisory control scheme. Dr. Mohammad H. Salah Page no. 137
  • 139. ICS Key Components 1. Control Components Programmable Logic Controller (PLC): PLCs are also implemented as the primary components in smaller control system configurations. PLCs have a userprogrammable memory for storing instructions for the purpose of implementing specific functions such as I/O control, logic, timing, counting, three mode proportionalintegral-derivative (PID) control, communication, arithmetic, and data and file processing. ICS Key Components 1. Control Components Intelligent Electronic Devices (IED): An IED is a “smart” sensor/actuator containing the intelligence required to acquire data, communicate to other devices, and perform local processing and control. An IED could combine an analog input sensor, analog output, low-level control capabilities, a communication system, and program memory in one device. The use of IEDs in SCADA and DCS systems allows for automatic control at the local level. Dr. Mohammad H. Salah Page no. 138
  • 140. ICS Key Components 1. Control Components Human-Machine Interface (HMI): The HMI is software and hardware that allows human operators to monitor the state of a process under control, modify control settings to change the control objective, and manually override automatic control operations in the event of an emergency. The HMI also allows a control engineer or operator to configure set points or control algorithms and parameters in the controller. The HMI also displays process status information, historical information, reports, and other information to operators, administrators, managers, business partners, and other authorized users. ICS Key Components 1. Control Components Data Historian: The data historian is a centralized database for logging all process information within an ICS. Information stored in this database can be accessed to support various analyses, from statistical process control to enterprise level planning. Input/Output (IO) server: The IO server is a control component responsible for collecting, buffering and providing access to process information from control subcomponents such as PLCs, RTUs and IEDs. An IO server can reside on the control server or on a separate computer platform. IO servers are also used for interfacing third-party control components, such as an HMI and a control server. Dr. Mohammad H. Salah Page no. 139
  • 141. ICS Key Components 2. Network Components Fieldbus Network: It links sensors and other devices to a PLC or other controller. Use of fieldbus technologies eliminates the need for point-to-point wiring between the controller and each device. The sensors communicate with the fieldbus controller using a specific protocol. The messages sent between the sensors and the controller uniquely identify each of the sensors. Control Network: The control network connects the supervisory control level to lower-level control modules. ICS Key Components 2. Network Components Communications Routers: A router is a communications device that transfers messages between two networks. Common uses for routers include connecting a LAN to a WAN, and connecting MTUs and RTUs to a long-distance network medium for SCADA communication. Remote Access Points: Remote access points are distinct devices, areas and locations of a control network for remotely configuring control systems and accessing process data. Examples include using a personal digital assistant (PDA) to access data over a LAN through a wireless access point, and using a laptop and modem connection to remotely access an ICS system. Dr. Mohammad H. Salah Page no. 140
  • 142. ICS Key Components 2. Network Components Firewall: A firewall protects devices on a network by monitoring and controlling communication packets using predefined filtering policies. Firewalls are also useful in managing ICS network isolation strategies. Modems: A modem is a device used to convert between serial digital data and a signal suitable for transmission over a telephone line to allow devices to communicate. Modems are often used in SCADA systems to enable long-distance serial communications between MTUs and remote field devices. They are also used in SCADA systems, DCS and PLCs for gaining remote access for operational and maintenance functions such as entering commands or modifying parameters, and diagnostic purposes. SCADA Systems SCADA is not a full control system, but rather focuses on the supervisory level. SCADA is used for gathering, analyzing and to storage real time data. SCADA systems consist of both hardware and software. Typical hardware includes an MTU placed at a control center, communications equipment (e.g., radio, telephone line, cable, or satellite), and one or more geographically distributed field sites consisting of either an RTU or a PLC, which controls actuators and/or monitors sensors. The MTU stores and processes the information from RTU inputs and outputs, while the RTU or PLC controls the local process Dr. Mohammad H. Salah Page no. 141
  • 143. SCADA Systems The communications hardware allows the transfer of information and data back and forth between the MTU and the RTUs or PLCs. The software is programmed to tell the system what and when to monitor, what parameter ranges are acceptable, and what response to initiate when parameters change outside acceptable values. An IED, such as a protective relay, may communicate directly to the SCADA Server, or a local RTU may poll the IEDs to collect the data and pass it to the SCADA Server. IEDs provide a direct interface to control and monitor equipment and sensors. SCADA Systems IEDs may be directly polled and controlled by the SCADA Server and in most cases have local programming that allows for the IED to act without direct instructions from the SCADA control center. SCADA systems are usually designed to be fault-tolerant systems with significant redundancy built into the system architecture. SCADA System General Layout (Components and General Configuration) Dr. Mohammad H. Salah Page no. 142
  • 144. SCADA Systems The control center houses a SCADA Server (MTU) and the communications routers. Other control center components include the HMI, engineering workstations, and the data historian, which are all connected by a LAN. The control center collects and logs information gathered by the field sites, displays information to the HMI, and may generate actions based upon detected events. The control center is also responsible for centralized alarming, trend analyses, and reporting. The field site performs local control of actuators and monitors sensors. SCADA Systems Field sites are often equipped with a remote access capability to allow field operators to perform remote diagnostics and repairs usually over a separate dial up modem or WAN connection. Standard and proprietary communication protocols running over serial communications are used to transport information between the control center and field sites using telemetry techniques such as telephone line, cable, fiber, and radio frequency such as broadcast, microwave and satellite. Dr. Mohammad H. Salah Page no. 143
  • 145. SCADA Systems MTU-RTU communication architectures vary among implementations. The various architectures used, including point-to-point, series, series-star, and multi-drop. Point-to-point is functionally the simplest type; however, it is expensive because of the individual channels needed for each connection. In a series configuration, the number of channels used is reduced; however, channel sharing has an impact on the efficiency and complexity of SCADA operations. Similarly, the series-star and multi-drop configurations’ use of one channel per device results in decreased efficiency and increased system complexity. SCADA Systems Basic SCADA Communication Topologies Dr. Mohammad H. Salah Page no. 144
  • 146. SCADA Systems The four basic architectures can be further augmented using dedicated communication devices to manage communication exchange as well as message switching and buffering. Large SCADA systems, containing hundreds of RTUs, often employ sub-MTUs to alleviate the burden on the primary MTU. This type of topology is shown in the following figure. SCADA Systems Large SCADA Communication Topology Dr. Mohammad H. Salah Page no. 145
  • 147. SCADA Systems SCADA System Implementation Example (Distribution Monitoring And Control) SCADA Systems This particular SCADA system consists of a primary control center and three field sites. A second backup control center provides redundancy in the event of a primary control center malfunction. Point-to-point connections are used for all control center to field site communications, with two connections using radio telemetry. The third field site is local to the control center and uses the wide area network (WAN) for communications. A regional control center resides above the primary control center for a higher level of supervisory control. Dr. Mohammad H. Salah Page no. 146
  • 148. SCADA Systems The corporate network has access to all control centers through the WAN, and field sites can be accessed remotely for troubleshooting and maintenance operations. The primary control center polls field devices for data at defined intervals (e.g., 5 seconds, 60 seconds) and can send new set points to a field device as required. In addition to polling and issuing high-level commands, the SCADA server also watches for priority interrupts coming from field site alarm systems. SCADA Systems SCADA System Implementation Example (Rail Monitoring and Control) Dr. Mohammad H. Salah Page no. 147
  • 149. SCADA Systems The previous example includes a rail control center that houses the SCADA system and three sections of a rail system. The SCADA system polls the rail sections for information such as the status of the trains, signal systems, traction electrification systems, and ticket vending machines. This information is also fed to operator consoles at the HMI station within the rail control center. The SCADA system also monitors operator inputs at the rail control center and disperses high-level operator commands to the rail section components. SCADA Systems In addition, the SCADA system monitors conditions at the individual rail sections and issues commands based on these conditions (e.g., shut down a train to prevent it from entering an area that has been determined to be flooded or occupied by another train based on condition monitoring). Dr. Mohammad H. Salah Page no. 148
  • 150. SCADA Systems There are several common media of communication: - Fiber optics - Electrical cable. - Leased lines from a telephone utility. - Satellite telecommunications. The communications method used by most SCADA systems is called “master–slave”, where only one of the machines (in this case the MTU) is capable of initiating communication. The MTU talks to each RTU then returns to the first. This is called "scanning". The time required for the MTU to scan ALL its RTUs is called the MTU Scan Time (Scan Interval). Factors that determine scan interval are: number of RTUs, amount of data, data rate, and communications efficiency. Example SCADA Systems Calculate a scan interval for a SCADA system that: - Has 20 RTUs - Every RTU has a point count of 180 status points, 30 alarm points, 10 meters (at 16 bits each), and 10 analog points (at 16 bits each). - The MTU sends information to the RTU of 150 discrete control signals to valves and motors, 6 stepping motors (16 bits each), and 10 valve controller set points (16 bits each) - Data rate for communication is 1200bps. - Communication efficiency is 40%. Solution Total Points is 920, therefore the total amount of data is 20 x 920 = 18,400bits and the data rate is 18,400b/1200bps =~ 15sec at 100% efficiency but at 40% efficiency, the scan interval is 15sec/0.4 =~ 38sec. Dr. Mohammad H. Salah Page no. 149
  • 151. DCS In a DCS, the data acquisition and control functions are performed by a number of distributed microprocessor-based units, situated near to the devices being controlled or, the instrument from which data is being gathered. DCS systems have evolved into providing very sophisticated analogue (e.g. loop) control capability. A closely integrated set of operator interfaces (or man machine interfaces) is provided to allow for easy system configurations and operator control. The data highway is normally capable of high speeds - typically 1 Mbps up to 10 Mbps. DCS Dr. Mohammad H. Salah Page no. 150
  • 152. DCS The PLC is still one of the most widely used control systems in industry. As needs grew to monitor and control more devices in the plant, the PLCs were distributed and the systems became more intelligent and smaller in size. PLCs and DCS are used as shown Remote Terminal Units (RTUs) RTU (sometimes referred to as a remote telemetry unit) as the title implies, is a microprocessor controlled electronic device which interfaces objects in the physical world to a distributed control system (DCS) or SCADA system by transmitting telemetry data to the system and/or altering the state of connected objects based on control messages received from the system RTU is a standalone data acquisition and control unit, generally microprocessor based, which monitors and controls equipment at some remote location from the central station. Its primary task is to control and acquire data from process equipment at the remote location and to transfer this data back to a central station. Dr. Mohammad H. Salah Page no. 151
  • 153. Remote Terminal Units (RTUs) It generally also has the facility for having its configuration and control programs dynamically downloaded from some central station. There is also a facility to be configured locally by some RTU programming unit. Remote Terminal Units (RTUs) Although traditionally the RTU communicates back to some central station, it is also possible to communicate on a peerto-peer basis with other RTUs. The RTU can also act as a relay station (sometimes referred to as a store and forward station) to another RTU, which may not be accessible from the central station. Small sized RTUs generally have less than 10 to 20 analog and digital signals, medium sized RTUs have 100 digital and 30 to 40 analog inputs. RTUs, having a capacity greater than this can be classified as large. Dr. Mohammad H. Salah Page no. 152
  • 154. Remote Terminal Units (RTUs) RTU is a device installed at a remote location that: 1. Collects data 2. Codes data into a format that is transmittable 3. Transmits the data back to a central station (master) 4. Collects information from the master device and implements processes that are directed by the master RTU is equipped with input channels for sensing and output channels for control or alarms and communications port. RTU - Types There are two basic types of RTU 1. The “single board RTU” which is compact, and contains all I/O on a single board 2. The “modular RTU” which has a separate CPU module, and can have other modules added, normally by plugging into a common “backplane” (a bit like a PC motherboard and plug in peripheral cards). The single board RTU normally has fixed I/O (e.g., 16 digital inputs, 8 digital outputs, 8 analogue inputs, and say 4 analogue outputs). It is normally not possible to expand its capability. The modular RTU is designed to be expanded by adding additional modules. Typical modules may be a 8 analog in module, a 8 digital out module. Some specialized modules such as a GPS time stamp module may be available. Dr. Mohammad H. Salah Page no. 153
  • 155. RTU - Sizes Tiny stand-alone systems that run off batteries for an entire year or more. These systems log data into EPROM or FLASH ROM and download data when physically accessed by an operator. Often these systems use single chip processors with minimal memory and might not be able to handle a sophisticated communications protocol. Small stand-alone systems that can power up periodically and apply power to sensors (or radios) to measure and/or report. Usually run off batteries that are maintained by solar energy. The batteries are large enough to maintain operation for at least 4 months during the darkness of the winter in the far northern hemisphere. These systems generally have enough capability for a much more complex communications scheme. RTU - Sizes Medium Systems that are dedicated single board industrial computers, including IBM-PC or compatible computers either in desk-top enclosures or industrial configurations such as VME, MultiBus, STD bus, PC104, etc…. Large Systems for complete Plant control with all the bells and whistles. These are usually in Distributed Control Systems (DCSs) in Plants, and often communicate over high speed LANs. Timing may be very critical. Dr. Mohammad H. Salah Page no. 154
  • 156. RTU – Architecture and Communications The SCADA RTU has the following hardware features: 1. CPU and volatile memory 2. Non volatile memory for storing programs and data 3. Communications capability either through serial port(s) or sometimes with an on board modem 4. Secure Power supply (with battery backup) 5. Watchdog timer (to ensure the RTU restarts if something fails) 6. Electrical protection against "spikes" 7. I/O interfaces to DI/DO/AI/AO's 8. Real time clock RTU – Architecture and Communications Dr. Mohammad H. Salah Page no. 155
  • 157. RTU – Architecture and Communications RTU monitors the field digital and analog parameters and transmits all the data to the Central Monitoring Station. RTU can be interfaced with the Central Station with different communication media (usually serial (RS232, RS485, RS422) or Ethernet) RTU can support standard protocols (Modbus, DNP3, ICCP…etc.) to interface any third party software. In some control application, RTU drives high current capacity relays to a digital output board to switch power on and off the devices in the field RTU – Architecture and Communications RTU can monitor Analog inputs that can be of different types like (4 to 20mA), (0 to 10V), (-2.5 to 2.5V), (1 to 5V)…etc RTU then translates this raw data into the appropriate units (e.g., gallons of water or temperature) before presenting the data to the user via the Human Computer Interface (HCI) or Man-Machine Interface (MMI) Dr. Mohammad H. Salah Page no. 156
  • 158. RTU – Applications Oil and Gas remote instrumentation monitoring, (offshore platforms, onshore oilwells) Networks of remote pump stations Hydro-graphic monitoring and control, (water supply, reservoirs, sewerage systems). Environmental monitoring systems (pollution, air quality, emissions monitoring). Minesite monitoring applications. Protection supervision and data logging of Power transmission network Air traffic equipments such as navigation aids. RTU – Comparison A PLC is a small industrial computer which originally replaced relay logic. It had inputs and outputs similar to those an RTU has. It contained a program which executed a loop, scanning the inputs and taking actions based on these inputs. Originally the PLC had no communications capability, but they began to be used in situations where communications was a desirable feature. So communications modules were developed for PLC's, supporting ethernet (for use in DCSs) and the Modbus communications protocol for use over dedicated (wire) links. As time goes on we will see PLC's support more sophisticated communications protocols. Dr. Mohammad H. Salah Page no. 157
  • 159. RTU – Comparison RTU's have always been used in situations where the communications are more difficult, and the RTU's strength was its ability to handle difficult communications. RTU's originally had poor programmability in comparison to PLC's. As time has gone on, the programmability of the RTU has increased. We are seeing the merging of RTU's and PLC's, but it will be a long time (if ever) before the distinction disappears.. RTUs, PLCs and DCS are increasingly beginning to overlap in responsibilities, and many vendors sell RTUs with PLC-like features and vice versa. The industry has standardized for creating programs to run on RTUs and PLCs RTU – Comparison RTU differs from a PLC in that RTUs are more suitable for wide geographical telemetry, often using wireless communications, while PLCs are more suitable for local area control (plants, production lines, etc.) where the system utilizes physical media for control Some vendors now supply RTUs with comprehensive functionality pre-defined, sometimes with PLC extensions and/or interfaces for configuration Some suppliers of RTUs have created simple Graphical User Interfaces (GUI) to enable customers to configure their RTUs easily Dr. Mohammad H. Salah Page no. 158
  • 160. Telemetry Telemetry refers to the transfer of remote measurement data to a central control station over a communications link. This measurement data is normally collected in real-time (but not necessarily transferred in real-time). The terms SCADA, DCS, PLC and smart instrument are all applications of the telemetry concept. Modems The telephone system, landline communication systems, and radio systems cannot directly transport digital information without some distortion in the signal due to the bandwidth limitation inherent in the connecting medium. A conversion device, called a modem (modulator/demodulator), is thus required to convert the digital signals into an analog form suitable for transmission over a telephone network. This converts the digital signals generated by a computer into an analog form suitable for long distance transmission over the cable or radio system. The demodulation portion of the modem receives this analog information and converts it back into the original digital information generated by the transmitting computer. Dr. Mohammad H. Salah Page no. 159
  • 161. Modems Modems - Types There are two types of modem available today: Dumb (or non-intelligent) modems depend on the computer to which they are connected, to instruct the modem when to perform most of the tasks such as answering the telephone. Smart modems have an on-board microprocessor enabling them to perform such functions as automatic dialing and the method of modulation to use. Dr. Mohammad H. Salah Page no. 160
  • 162. Modems – Communication Protocols Modems can be either synchronous or asynchronous. In asynchronous communications each character is encoded with a start bit at the beginning of the character bit stream and a parity and stop bit at the end of the character bit stream. The receiver then synchronizes with each character received by looking out for the start bit. Once the character has been received, the communications link returns to the idle state and the receiver watches out for the next start bit (indicating the arrival of the next character). Modems Dr. Mohammad H. Salah Page no. 161
  • 163. Modems Synchronous communication relies on all characters being sent in a continuous bit stream. The first few bytes in the message contain synchronization data allowing the receiver to synchronize onto the incoming bit stream. Hereafter synchronization is maintained by a timing signal or clock. The receiver follows the incoming bit stream and maintains a close synchronization between the transmitter clock and receiver clock. Synchronous communications provides for far higher speeds of transmission of data, but is avoided in many systems because of the greater technical complexity of the communications hardware. Modems Dr. Mohammad H. Salah Page no. 162
  • 164. Modems – Modes of Operation Modems can operate in three modes: Simplex Half-Duplex Full-Duplex A simplex system in data communications is one that is designed for sending messages in one direction only and has no provision for sending data in the reverse direction. Modems – Modes of Operation A duplex system in data communications is one that is designed for sending messages in both directions. Duplex systems are said to be half-duplex when messages and data can flow in both directions but only in one direction at a time. Duplex systems are said to be full-duplex when messages can flow in both directions simultaneously. Full-duplex is more efficient, but requires a communication capacity of at least twice that of half-duplex. Dr. Mohammad H. Salah Page no. 163
  • 165. Modems – Interface Standards RS-232, RS-422 and RS-485 standards form the key element in transferring digital information between the RTUs (or operator terminals), and the modems, which convert the digital information to the appropriate analog, form suitable for transmission over greater distances. The RS-232 standard was initially designed to connect digital computer equipment to a modem where the data would then be converted into an analog form suitable for transmission over greater distances. The RS-422 and RS-485 standards can perform the same function but also have the ability of being able to transfer digital data over distances of over 1200 m. The most popular (but probably technically the most inferior (poorer)) of the RS standards is the RS-232C standard. Modems – RS232C Interface Standard The RS-232 interface standard was developed to interface between data terminal equipment (DTE) and data communications equipment (DCE) employing serial binary data interchange. The EIA-RS-232 standard consists of 3 major parts, which define: - Electrical signal characteristics - Interface mechanical characteristics - Functional description of the interchange circuits Dr. Mohammad H. Salah Page no. 164
  • 166. Modems – RS232C Interface Standard Electrical signal characteristics: Electrical signals such as the voltage levels and grounding characteristics of the interchange signals and associated circuitry. Interface mechanical characteristics: It dictates that the interface must consist of a ‘plug’ and Modems 183 ‘receptacle’ (socket) and that the receptacle will be on the DCE. In RS-232C, the pin number assignments are specified but, originally, the type of connector was not. Functional description of the interchange circuits This defines the function of the data, timing, and control signals used at the interface between DTE and DCE. Modems – RS232 Limitations The restriction of point-to-point communications is a drawback when many devices have to be multi-dropped together. The distance limitation (typically 15 meters) is a limitation when distances of 1000m are needed. The 20 kbps baud rate is too slow for many applications. The voltages of –3 to –25 volts and +3 to +25 volts are not compatible with many modern power supplies (in computers) of +5 and +12 volt. The standard is an example of an unbalanced standard with high noise susceptibility. Two approaches to deal with the limitations of RS-232 are the RS-422 and RS-485 standards. Dr. Mohammad H. Salah Page no. 165
  • 167. Modems – RS422 Interface Standard The RS-422 standard introduced in the early 70s defines a differential data communications interface using two separate wires for each signal This permits very high data rates and minimizes problems with varying ground potential because the ground is not used as a voltage reference (in contrast to RS-232) and allows reliable serial data communication for: - Distances of up to 1200 m - Data rates of up to 10 Mbps - Only one line driver is permitted on a line - Up to 10 line receivers can be driven by one line driver Modems – RS422 Interface Standard The line voltages range between –2 V to –6 V for Logic 1 and +2 V to +6 V for Logic 0 (using terminals A and B as reference points). The line driver for the RS-422 interface produces a ±5 V differential voltage on two wires. The two signaling states of the line are defined as follows: - When the ‘A’ terminal of the driver is negative with respect to the ‘B’ terminal, the line is in a binary 1 (Mark or Off) state. - When the ‘A’ terminal of the driver is positive with respect to the ‘B’ terminal, the line is in a binary 0 (Space or On) state. Dr. Mohammad H. Salah Page no. 166
  • 168. Modems – RS422 Interface Standard The differential voltage signal is the major feature of the RS422 standard, which allows an increase in speed and provides higher noise immunity. Each signal is transferred on one pair of wires and is the voltage difference between them. A common ground wire is preferred to aid noise rejection. Consequently, for a full-duplex system, five wires are required (with 3 wires for half-duplex systems). The RS-422 standard does not specify the mechanical connections or assign pin numbers and leaves this aspect optional. Modems – RS485 Interface Standard The RS-485 standard is the most adaptable and flexible. It is an expansion of RS-422 and allows the same distance and data speed but increases the number of transmitters and receivers permitted on the line. RS-485 permits multi-drop network connection on two wires and provides for reliable serial data communication for: - Distances of up to 1200 m (same as RS-422) - Data rates of up to 10 Mbps (same as RS-422) - Up to 32 line drivers permitted on the same line - Up to 32 line receivers permitted on the same line Dr. Mohammad H. Salah Page no. 167
  • 169. Modems – RS485 Interface Standard The line voltages are similar to RS-422 ranging between –1.5V to –6V for logic ‘1’ and +1.5V to +6V for Logic ‘0’. As with RS-422, the line driver for the RS-485 interface produces a 5 volt differential voltage on two wires. For full-duplex systems, five wires are required. For a half-duplex system, only three wires are required. The major enhancement of RS-485 is that a line driver can operate in three states (called tri-state operation) logic ‘0’, logic ‘1’ and ‘high-impedance’, where it draws virtually no current and appears not to be present on the line. This latter state is known as the ‘disabled’ state and can be initiated by a signal on a control pin on the line driver integrated circuit. Modems – RS485 Interface Standard The RS-485 interface standard is useful where distance and connection of multiple devices on the same pair of lines is desirable. Special care must be taken with the software to coordinate which devices on the network can become active. In most cases, a master terminal, such as a PLC or computer, controls which transmitter/receiver will be active at any one time. Dr. Mohammad H. Salah Page no. 168
  • 170. SCADA Examples – Water Treatment SCADA Examples – Wind Farms 1 Dr. Mohammad H. Salah Page no. 169
  • 171. SCADA Examples – Wind Farms 2 SCADA Examples – Cement Mill Dr. Mohammad H. Salah Page no. 170
  • 172. SCADA Examples – Level Control Dr. Mohammad H. Salah Page no. 171

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