1. PLCs were invented in answer to the needs of the automobile industry, and were introduced in the late 1960’s. They were designed to take the place of older, more complicated relay based equipment systems. ｩ Emil Decker, 2009

2. Relays were invented in 1835 by Joseph Henry. Relays use an electro- magnet to close or open a switch. This circuit is isolated from the power that runs through the main circuit involved. ｩ Emil Decker, 2009

3. This allows a low voltage relay to “throw the switch” on a higher voltage line and keep the operator safe. Relays are used in many electrical systems, including automobiles. ｩ Emil Decker, 2009

4. Relays are complicated, and systems using a lot of them tended to be very costly to operate and maintain. If production requirements changed, then the control system had to change also. ｩ Emil Decker, 2009

5. The mechanical relays had a limited lifespan, and troubleshooting them were often problematic. Imagine a system using literally thousands of relays. Typical Relay ｩ Emil Decker, 2009

6. By replacing bulky mechanical components with solid state circuits under the control of a microcontroller makes a lot of sense. ｩ Emil Decker, 2009

7. The microprocessor is the brain of a PLC system. It consists of the microprocessor, memory, and circuits necessary to store and retrieve data from that memory. It also includes communications ports to other devices. ｩ Emil Decker, 2009

8. Processors vary in their capabilities to control devices. Some control as few as 6 inputs and outputs (I/O) and others control as many as 40,000 or more. ｩ Emil Decker, 2009

9. One processor can control more than one process or manufacturing line. Processors are often linked together in order to provided continuity throughout the process. ｩ Emil Decker, 2009

10. Ram, or random access memory, is necessary for PLCs. Like computers, this is the area where all instructions are stored during operations. Everything happens in RAM. ｩ Emil Decker, 2009

11. ROM, or read only memory, is important too. Instructions burned into ROM cannot be changed. This is where the basic program is stored. You load this and your routines into RAM to run them. ｩ Emil Decker, 2009

12. EEPROM, or Electrically Erasable Programmable Read Only Memory, is like ROM, but with the correct equipment, can be reprogrammed, making it more versitile. ｩ Emil Decker, 2009

13. Input modules are simply what they say they are. The input signals, whether via a push button, limit switch, electric eye, or an electrical input. ｩ Emil Decker, 2009

14. If there are input modules, then there must be output modules as well. LED readouts, solenoids, relays, DC or AC outputs to machinery, are a few options you have for output. ｩ Emil Decker, 2009

15. PLCs, at their simplest, are ON / OFF switches, sequentially operated to send signals to an output device. ｩ Emil Decker, 2009

16. Electromagnetic interference (EMI) and Radio Frequency Interference (RFI) can cause problems in most solid state control systems, and PLCs are not immune. ｩ Emil Decker, 2009

17. PLCs are used in a variety of applications from manufacturing assembly lines to equipment control in the harshest of environments. Coal mining machine controlled by PLCs ｩ Emil Decker, 2009

18. PLC input modules PLC processor PLC Output modules Switches Sensors Machine outputs Other outputs PLC Communications modules PLC special Purpose modules Programmable Logic controller PLC Power supply Standard networks Proprietary networks Smart I/O interfaces Remote racks ASCII I/O interfaces Lamps Relays Motor controllers Solenoid valves Machine inputs Other inputs ｩ Emil Decker, 2009