This document provides an introduction to programmable logic controllers (PLCs), including a brief history of their evolution from relay-based industrial control systems. It describes the basic architecture of a PLC system and its advantages over relay controls, such as modularity, reprogrammability, communication capabilities, and reduced installation costs. The document also outlines common PLC modules, programming languages, and methods for addressing inputs and outputs.

1. Introduction to Programmable Logic
Controller (PLC)
By
Dr Prasanta Roy
Assistant Professor
Department of Electrical Engineering, NIT Silchar

2. Contents
1. Evolution of PLC
2. Advantage of PLC over Relay Control
3. Architecture of PLC
4. Functions of PLC
5. Various Modules of PLC
6. Addressing of I/Os
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3. What is PLC?
A special purpose industrial microprocessor based real-time
computing system, which performs the following functions in
the context of industrial operations.
• Monitor Input/Sensors
• Execute logic, sequencing, timing, counting functions for
Control/Diagnostics
• Drives Actuators/Indicators
• Communicates with other computers
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4. Sequence Control using Relays
(Pre-PLC Era)
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5. Evolution of PLC
Before the advent of microprocessors
• Industrial logic and sequence control used to be performed
using elaborate control panels containing relays, contactors
and switches, indicator lamps, mechanical or electronic
timers and counters etc.
• All hardwired by complex and elaborate wiring.
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6. A brief history of PLC
• It came into existence in 1968 in US by General Motors
• PLC for domestic application started in 1970 in Japan
• General purpose PLCs came in 1976
• Current Manufacturer: Allen Bradley, ABB, Siemens,
Omron, Mitsubishi, Hitachi, Delta, General Electric,
Honeywell, Modicon, Schneider Electric, Bosch etc.
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7. Advantage of PLC over Relay control
1. Simple modular assembly and connection
2. Modular expansion capacity of the I/O and memory
3. Flexible Re-programming environments
4. Standardized task libraries and debugging aids
5. Communication capability with other programmable
controllers and computers
6. Takes much less space
7. Installation and maintenance is easier resulting in greater
reliability.
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8. Advantage of PLC over Relay control
8. Can be connected easily to a distributed plant automation
system, supervised and monitored.
9. Ability to accept digital data in serial, parallel and
network modes
But Some Legacies Continued..
To facilitate maintenance and modification of the physically
wired control logic, the control panel was organized like a
rung on a ladder.
• PLCs retained the ladder logic concept
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9. Architecture of a PLC System
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10. Optocoupler
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11. PLC System
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12. PLC Module mounted in rack
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13. Various type of PLCs
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14. Q Series PLC (Mitsubishi Electric)
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15. Q-Series PLC (ME)
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16. Fx-Series PLC (ME)
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17. PLC Software Environment
• Siemens PLC- SIMATIC S7
• Mitsubishi PLC - GX Works
• Allen Bradley PLC- RSLOGIX
• ABB PLC - Automation Builder
• Bosch -Bosch Rexroth
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18. Supported programming language PLC
Software Environment
IEC 1131 is an international standard for PLCs formulated by
the International Electrotechnical Commission (IEC). It
specifies the syntax, semantics and graphics symbols for the
following PLC programming languages:
• Relay Ladder Logic (RLL)
• Sequential Function Charts (SFC)
• Function Block Diagram (FBD)
• Structured Text (ST)
• Instruction List (IL)
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19. PLC Software Environment
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20. Functions of a PLC
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21. Functions of a PLC
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22. How a PLC program run?
• After the PLC is initialised, the processor reads the
individual inputs. This status of the input is stored
in the process- image input (PII) table.
• The logical functions and instructions are
successively processed based on the data on PII
and the results are written into a process-image
output (PIQ) table.
• The status from the PIQ will transfer to the outputs
and then it begins the execution of the next cycle.
• The time required to complete one cycle is known
as the Scan Time.
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23. Power Supply Module (ME Q Series)
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24. Basic CPU (ME Q Series)
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25. Basic CPU (ME Q Series)
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26. Redundant CPU (ME Q Series)
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27. Discrete/Digital I/O Modules (ME Q Series)
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28. Discrete/Digital I/O Modules (ME Q Series)
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29. Digital Input Modules (ME Q Series)
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30. Digital Output Modules (ME Q Series)
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31. Special function modules (ME Q Series)
(Analog input module)
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32. Special function modules (ME Q Series)
(Analog output module)
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33. Special function modules (ME Q Series)
(Temperature control module)
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34. Special function modules (ME Q Series)
(High Speed counter module)
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35. Special function modules (ME Q Series)
(Position controller module)
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36. Special function modules (ME Q Series)
(Serial communication module)
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37. Special function modules (ME Q Series)
(CC-link Module)
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38. Special function modules (ME Q Series)
(Ethernet Interface Module)
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39. Special function modules (ME Q Series)
(MELSECNET Module)
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40. Addressing of I/O
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41. Addressing of I/O
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42. Addressing of I/O
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43. An Exercise: Addressing of I/O
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44. Solution: Addressing of I/O
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45. Thank You
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