3. What is a PLC ?
• PLC (Programmable Logic Controller)
• A PLC works by looking at its inputs
and depending on their state, and the
user entered program, turns on/off
outputs.
• A PLC can be thought of as:
Industrial Computers with specially
designed architecture in both their
central units (the PLC itself) and their
interfacing circuitry to field devices
(input / output connections to the real
world).
6. CPU Module
• The Central Processing Unit (CPU)
Module is the brain of the PLC.
• Primary role to read inputs, execute
the control program, update outputs.
• The CPU consists of the arithmetic
logic unit (ALU), timing/control
circuitry, accumulator, scratch pad
memory, program counter, address
stack and instruction register.
• A PLC works by continually scanning
a program
Self
Check
Execute
Code
Scan
Inputs
Update
Outputs
PLC Program
SCAN
7. Memory
• The memory includes pre-programmed ROM memory
containing the PLC’s operating system, driver
programs and application programs and the RAM
memory.
• PLC manufacturer offer various types of retentive
memory to save user-programs and data while power
is removed, so that the PLC can resume execution of
the user-written control program as soon as power is
restored.
8. I/O Modules
• Input and output (I/O) modules connect
the PLC to sensors and actuators.
• Provide isolation for the low-voltage, low-
current signals that the PLC uses
internally from the higher-power
electrical circuits required by most
sensors and actuators.
• Wide range of I/O modules available
including: digital (logical) I/O modules
and analog (continuous) I/O modules.
9. Inputs Modules
• Inputs come from sensors that translate physical or
chemical phenomena into electrical signals.
• The simplest form of inputs are digital/discrete in AC/DC.
• In smaller PLCs the inputs are normally built in and are
specified when purchasing the PLC.
• For larger PLCs the inputs are purchased as modules, or
cards, with 8,16, 32, 64, 96 inputs of the same type on
each card.
10. Inputs Modules
The list below shows typical ranges for input voltages.
5 Vdc
12 Vdc
24 Vdc
48 Vdc
12 Vac
24 Vac
120 Vac
240 Vac
12. Outputs Modules
• Output modules rarely supply any power, but instead act
as switches.
• External power supplies are connected to the output card
and the card will switch the power on or off for each
output.
• A common choice when purchasing output cards is
relays, transistors or triacs.
• Relay are the most flexible output devices. They are
capable of switching both AC and DC outputs. But, they
are slower, cost more, and they will wear out after
millions of cycles.
13. Relays
• The most important consideration
when selecting relays, or relay
outputs on a PLC, is the rated
current and voltage.
• For transistor outputs or higher
density output cards relay terminal
blocks are available.
– Advantage of individual standard
replaceable relays
14. Outputs
Typical output voltages are listed below,
5 Vdc
12 Vdc
24 Vdc
48 Vdc
24 Vac
120 Vac
240 Vac
WARNING: Always check rated voltages and currents for
PLCs and never exceed.
17. Analogue Inputs/Outputs
• Analogue input cards convert continuous signals via a
A/D converter into discrete values for the PLC
• Analogue output cards convert digital values in then PLC
to continuous signals via a D/A converter.
• Resolution can be important in choosing an applicable
card
• Example, for a temperature input of 0 to 100 degrees C
– For 8 bit resolution the value in the PLC is 0 to 255
– For 12 bit resolution the value in the PLC is 0 to 4095
– For 12.5 bit resolution the value in the PLC is 0 to 6000
– For 13 bit resolution the value in the PLC is 0 to 8192
– For 16 bit resolution the value in the PLC is 0 to 32768
18. Special Modules
• RF ID
• Voice
• Gas Flow Calculation
• Weigh Cell
• Hydraulic Servo
• ASCII
• Fuzzy Logic
• Temperature Sensor
• Temperature Control
• Heat/Cool Control
• Field Bus Cards
– DeviceNet, Profibus etc
– Lonworks, BACNet
• Fast Response (Interrupt)
• PID
• Loop Controller
• BASIC Cards
• RS232 Comm’s
• Modbus ASCII/RTU
• Ethernet Comm’s
• High Speed Counters
• Position Control Cards
• Per to Per Comm’s
– Controller Link
– DH+
– Modbus Plus
19. Available Instructions
• Sequence
– Input
– Output
– Control
• Logic
• Timer and Counters
• Comparison
• Range Comparison
• Data Movement
• Data Shift
• Step / Step Next
• Serial Communications
• Text String Processing
• File Manipulation
• Increment/Decrement
• Conversion
– ASCII
– Number Systems
• Math
• Floating Point Math
• Statistics
• Scaling
• PID
• PID with Auto tune
• Clock / Date
• Block Processing
– IF,THEN,ELSE,LOOP
Table Processing
– LIFO, FIFO
21. The Configuration of PLC
• The configuration of PLC refers to the packaging of the
components.
• Typical configurations are listed below from largest to
smallest.
– Rack Type : A rack can often be as large as 18” by 30” by 10”
– Mini: These are similar in function to PLC racks, but about the
half size. Dedicated Backplanes can be used to support the
cards OR DIN rail mountable with incorporated I/O bus in
module.
– Shoebox: A compact, all-in-one unit that has limited expansion
capabilities. Lower cost and compactness make these ideal for
small applications. DIN rail mountable.
– Micro: These units can be as small as a deck of cards. They tend
to have fixed quantities of I/O and limited abilities, but costs will
be lowest. DIN rail mountable.
22. Sizing of PLC
Micro PLCs: I/O up to 32 points
Small PLC: I/O up to 128 points
Medium PLC: I/O up to 1024 points
Large PLC: I/O up to 4096 points
Very Large: I/O up to 8192 points
23. Selecting a PLC
• Criteria
• Number of logical inputs and outputs
• Memory
• Number of special I/O modules
• Expansion Capabilities
• Scan Time
• Communication
• Software
• Support
• Dollars
