A PLC is a programmable device that was invented to replace sequential relay circuits for machine control. It scans its inputs and outputs and executes a user-programmed logic program to control outputs based on inputs. Early PLCs were programmed using ladder logic to resemble relay circuits. Modern PLCs can be programmed in various languages and communicate with other devices. A PLC consists of a CPU, memory, and input and output modules to interface with the outside world.

1. PLC Programmable Logic Controller

2. PLC A PLC is a device that was invented to replace the necessary sequential relay circuits for machine control The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs

3. PLC History In 1960's PLC’s were first introduced. Reason for designing such a device was to eliminate the large cost involved in replacing the complicated relay based machine control systems. Since relays are mechanical devices they also have a limited lifetime which required frequent maintenance schedules. Troubleshooting was also quite tedious when so many relays are involved. Now imagine a machine control panel that included many, possibly hundreds or thousands, of individual relays. Think about wiring & size.

4. PLC History The MODICON (Modular Digital Controller ) brought the world's first PLC into commercial production. Dick Morley , who worked in MODICON is considered to be the "father" of the PLC. These "new controllers" had to be easily programmed. The lifetime had to be long and programming changes easily performed. Early PLC’s were designed to replace relay logic systems. These PLC’s were programmed in " ladder logic ", which strongly resembles a schematic diagram of relay logic.

5. PLC History Modern PLC’s can be programmed in a variety of ways, from ladder logic to more traditional programming languages. Early PLC’s, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. Communications abilities began to appear in approximately 1973. The first such system was Modicon's Modbus. The PLC could now talk to other PLC’s.

6. Internal Structure Plc mainly consist of CPU MEMORY INPUT OUTPUT

7. Internal Structure INPUT RELAYS-these are connected to the outside world. They receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors. INTERNAL UTILITY RELAYS-these do not receive signals from the outside world. They are simulated relays and are what enables a PLC to eliminate external relays. There are also some special relays that are dedicated to performing only one task. COUNTERS-These do not physically exist. They are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down.

8. Internal Structure TIMERS-These also do not physically exist. They come in many varieties and increments. The most common type is an on-delay type. Others include off-delay. OUTPUT RELAYS-(coils) these are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. DATA STORAGE-Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation.

9. Operation A PLC works by continually scanning a program. We can think of this scan cycle as consisting of 3 important steps. Three states are. Check Input Status Execute Program Update Output Status

10. Scanning CHECK INPUT STATUS -First the PLC takes a look at each input to determine if it is on or off. In other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step. EXECUTE PROGRAM -Next the PLC executes your program one instruction at a time. Maybe your program said that if the first input was on then it should turn on the first output. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the state of the first input. It will store the execution results for use later during the next step.

11. Scanning UPDATE OUTPUT STATUS -Finally the PLC updates the status of the outputs. It updates the outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now turn on the first output because the first input was on and your program said to turn on the first output when this condition is true

12. Response Time INPUT - It took a certain amount of time for the brain to notice the input signal from the eyes. EXECUTION - It took a certain amount of time to process the information received from the eyes. If the eyes see an beautiful picture then output appropriate words to the mouth. OUTPUT - The mouth receives a signal from the brain and eventually spits out the words “wow!, that's a really beautiful picture!"

13. Relays We can think of a relay as an electromagnetic switch. Apply voltage to the coil, Due to the magnetic field generated, makes or breaks the circuit. These contacts can be considered to be a switch.

14. Relays When the switch is open no current can flow through the coil of the relay. As soon as the Trigger switch is closed, current runs through the coil causing a magnetic field to build up. This magnetic field causes the contacts of the relay to close. When Reset button is pressed, breaks the supply to the coil, makes the contact to open.

15. Ladder Diagram Let’s use a plc in place of the relay, for that you have to know about Ladder diagram. Power lines Two vertical bars, one on each side of the diagram. Left one is +ve and Right one is Ground. Input Output Coil / Contact

16. Ladder Diagram Input symbol Contact symbol can be used for real world switch (or) push button. It can also used for the Relay contacts. Output symbol Coil symbol can be used for the real world Relay coil. Contact symbol Coil symbol

17. Ladder Diagram We must tell the plc where everything is located. We have to give all the devices an address. Where is the switch going to be physically connected to the plc? Where is the output going to be physically present? The plc manufacturers each do it a different way. We have to convert the schematic into a logical sequence of events. The program tells the plc what to do when certain events take place.

18. Ladder Diagram

19. Gates Plc program's are made up of a combination of AND, OR, NAND, XOR,NOR Gates, with Inputs, Outputs, Timers, Counters, Memory Bits, Analog Inputs, analog Outputs etc.. Boolean math lets us do some vary basic functions with the bits in our registers. These basic functions typically include Gate (AND, OR, XOR) functions.

20. Gates - OR Here we can see that the OR function is very much related to Addition. A and B are Inputs C is Output C is True only when A or B is True C is also True when A and B also True.

21. Gates - OR Ladder diagram of OR Function Inputs “0” or “1” are connected in parallel. Any one or both Inputs On Output should be On. If both Inputs are Off Output should be Off. Ladder logic

22. Gates - AND Here we can see that the AND function is very much related to Multiplication. A and B are Inputs C is Output C is True only when A and B is True C is False when A or B is False.

23. Gates - AND Ladder Diagram of AND Function. Inputs “0” and “1” are connected in series. Both Inputs are On then Output should be On. Any one Input is Off then Output should be Off.

24. Gates - NOR NOR OR

25. Gates - NOR

26. Gates - XOR

27. Gates - XOR

28. Inputs These are the physical connections from the real world to the PLC. They can be limit switches, push buttons, sensors, anything that can "switch" a signal on or off. When voltage is present, Input will be “ON”, Status should be “1”. When voltage not present, Input will be “OFF”, Status should be “0”.

29. Outputs These are the connections from the PLC to the real world. They are used to switch solenoids, lamps, contactors etc ON and OFF. They are usually 24 Volt DC, either relay or transistor, but can also be 115/220 Volt AC.

30. Internal Memory These so called relays only exist in the PLC’s memory. They are used to remember certain "states" or functions that have been performed. They react exactly like ordinary relays and can be on, off or latching. Manufacturers of PLC’s also pre-program certain internal relays with special functions such as, Always on Always off On PLC is powering up On the first scan of the program On if there is an error

31. Counter A counter is a simple device intended to do one simple thing - count. What kinds of counters are there? Up Counters Down Counters Up Down Counters High Speed Counters High Speed Counters are hardware counters physically present in the PLC. Other counters are software counters, simulated through software, they are not physically present.

32. Counter Main things while selecting a Counter Where the pulses that we want to count are coming from. Typically this is from one of the inputs. How many pulses we want to count before we react. When/how we will reset the counter so it can count again. Typically counters can count from 0 to 9999 -32,768 to +32,767 0 to 65535.

33. Timer What is a timer? It’s exactly what the word says... It is an instruction that waits a set amount of time before doing something. Types of Timers On Delay Timer Off Delay Timer Retentive or Accumulating Timer

34. Timer On Delay Timer This type of timer simply "delays turning on". After our input turns on we wait x-seconds before activating a output. Off Delay Timer This timer simply "delays turning off". After our input turns off we wait x-seconds before deactivating a output.

35. Timer Retentive or Accumulating timer This type of timer needs 2 inputs. One input starts the timing event The other resets it. The on/off delay timers would be reset if the input sensor wasn't on/off for the complete timer duration. This timer however holds or retains the current elapsed time when the sensor turns off in mid-stream.

36. Timer What will enable the timer ? How long we want to delay before we react .