1. Welcome to the World of Programmable Logic Controllers
Chapter 1
Welcome to the World of Programmable Logic Controllers

2. Objectives Define PLC. Explain where the PLC came from.
Explain why their use is valuable.
Explain where they are used.
Detail what PLCs can do.
Explain how PLCs know what they are supposed to do.

3. What Is a Programmable Controller?
A programmable logic controller, usually called a PLC or programmable controller, is a solid-state, digital, industrial computer.
Simply, a programmable controller is a computer, much like a desktop personal computer.
A PLC is an industrially hardened computer.

4. Where Did the PLC Come From? (1 of 2)
In the 1960s, electromechanical relays, timers, counters, and sequencers were the standard.
Many control panels contained hundreds of these devices and a mile or more of wire.

5. Where Did the PLC Come From? (2 of 2)
Reliability was low and maintenance costs were high.
Cost was high to modify or upgrade control panels.
In 1968 the General Motors Hydramatic division specified a device that would become what we know today as the programmable logic controller.

6. Early PLCs Only relay replacers Did not have timers or counters
No sequencer instructions
No math instructions
No data manipulation instructions

7. Why a PLC? (1 of 2) Easily changeable Programmable Reliable Smaller
Fast switching

8. Why a PLC? (2 of 2) Able to withstand harsh factory environment
Consumes less power
Easier to troubleshoot
Easy to install

9. Why Use a PLC?
The question “why use a PLC?” should really be rephrased to “why automate?”
The PLC is the tool that provides the control for the automated process.

10. Automating Helps a Manufacturing Facility (1 of 2)
Gain complete control of the manufacturing process
Achieve consistency
Improve quality and accuracy
Work in difficult or hazardous environments
Increase productivity

11. Automating Helps a Manufacturing Facility (2 of 2)
Shorten lead time to market
Lower cost of quality, scrap, and rework
Offer greater product variety
Allow a quick changeover from one product to another
Control inventory

12. A PLC Upon First Glance
A black box with wires bringing signals in and other wires sending signals out
Some sort of magic being done inside that somehow decides when field devices should be turned on or off

13. Actually There Is No Magic
The PLC is a computer and someone has to tell it what to do.
The PLC knows what to do through a program that was developed and entered into its memory.
Without a set of instructions telling the PLC what to do, it is nothing more than a box full of electronic components.

14. What Makes a PLC Work? (1 of 4)
The heart of any computer is the microprocessor.
The microprocessor, also called the processor or central processing unit (CPU), supervises system control through the user program.

15. What Makes a PLC Work? (2 of 4)
The processor reads input signals and follows the instructions that the programmer has stored in the PLC’s memory.

16. What Makes a PLC Work? (3 of 4)
As a result of the solved program, the PLC writes information to outputs, or field-controlled devices, to turn them on or off.
When the PLC is running and following the program’s instructions, this is called solving the user program.
The PLC is running or in RUN MODE.

17. What Makes a PLC Work? (4 of 4)
The user program (ladder program) is the list of instructions that tells the PLC what to do.
The library of instructions available to the PLC is called the instruction set.
The instruction set determines how much flexibility the programmer has.

18. Common PLC Inputs Pushbuttons Selector switches
Limit switches and level switches
Proximity sensors
Photo switches
Relay contacts
Motor starter contacts

19. An Overview of a PLC System (1 of 2)

20. An Overview of a PLC System (2 of 2)
Incoming signals, or inputs, interact with instructions in the user program to help the PLC determine when an input instruction is either true or false.

21. Conventional Circuit

22. Representation of a PLC Program

23. PLC Ladder Program Rung

24. Series 90-30 and 90-20 Hand-held Programmer
Image courtesy of GE Fanuc Automation

25. Interfacing a PC to an Omron CQM 1 PLC
Image courtesy of Omron Electronics, Inc.

26. Correlating Ladder Program Rung to Actual PLC Wiring

27. Programmable Controller Block Diagram

28. Product Sensed in Position Will Send an Input Signal

29. Allen-Bradley SLC 500 Fixed PLC
Image courtesy of Allen-Bradley, a Rockwell Automation business

30. Allen-Bradley SLC 500 Modular PLC
Image courtesy of Allen-Bradley, a Rockwell Automation business

31. SLC 500 Power Supply and a Four-slot Rack
Image courtesy of Allen-Bradley, a Rockwell Automation business

32. Installation of an I/O Module
Image courtesy of Allen-Bradley, a Rockwell Automation business

33. Allen-Bradley SLC 500 Modular Processor
Image courtesy of Allen-Bradley, a Rockwell Automation business

34. Limit Switch Interface

35. Output Module Wiring to a Motor Starter Coil