1. Overview of the PLC

2. What is a PLC?
PLC is the short form of Programmable Logic Controller and is a microprocessor based device.

3. What is a PLC?
It is a programmable memory for storing instructions to implement specific functions such as
Logic
Sequencing
Timing
Counting
Interlocking to control machines and processors
More advanced PLC can also perform arithmetic and data manipulations

4. History of the PLC
PLC were designed to eliminate the high cost and inflexibility of relay control systems. When they designed plcs, they had these design criteria:
Solid-state with computer flexibility
Suited to industrial environment
Easily programmable
Easily maintained

5. The Early PLC Just relay replacers On/off control only
Limited to repetitive operations - transfer lines, grinding machines, boring machines, etc.
Improvement over relays
Easily installed
Uses less space and energy
Diagnostic indicators that aid troubleshooting
Reusable

6. Early Innovations
Microprocessor technology added greater flexibility and intelligence to PLC
Capable of operator interface, arithmetic, data manipulation and computer communications

7. Early Innovations
Cathode ray tube (CRT) programmer allowed relay symbol entry
Arithmetic function and improved instruction set allowed numerical input data for instrumentation devices
Logic and sequencing tasks enhanced

8. Later Innovations
Greater flexibility with hardware and software enhancements
Improvements included:
Larger memory capacity
Remote I/O
Analog and positioning control
Operator communications
Software enhancement
Reduction of wiring and installation costs

9. Controllers in the 80‘s
Early 1980s microprocessor technology brought about many advances
Keen competition affected design

10. Controllers in the 80‘s Hardware enhancements Faster scan times
Small, low cost and reduced space requirement
More I/O ports
Direct connection to devices
Improved mechanical design
Peripheral equipment improved operator interface and system documentation

11. Todays PLC Developments in hardware Advanced memory
User orientated operator interface
Universal programming device
Communication between different brands of plcs

12. Todays PLC Developments in software Iec 61131-3
Control strategies can be distributed instead of centralised
Integration with other control equipment such as robots

13. PLCs versus Relays I/O units - not directly connected to one another
Programmable - need only change program when circuit condition changes
Space saving - internal relays, timers, counters, etc.
Flexibility in control changes
Low power consumption
Easy debugging