Wherever there is an enterprise or industry, you will find a PLC.
The PLC has seen dramatic evolution since the first PLC, the Modicon, was introduced by Bedford Associates in the late 1960s (Kuphaldt, 2003). Paralleling the evolution of larger data processing systems, Allen Bradley and General Electric introduced programmable units with terminal input in the 1970s (Morley, 2006). They are now available as stand alone units, flexible card driven modules, and rack mounted with standardized cards for large scale centralized control. The unitary style is a self enclosed, ruggedised unit that has all the circuitry necessary to operate independently. It is generally used near the machine it is controlling and is dedicated to a small number of tasks. Modular construction has the advantage of placing several control units into a single module which allows the units to share power supply and computing capabilities. Sharing support circuitry reduces the cost of the unit as well as allowing for expanded control capabilities. Rack mounted systems are used to provide centralized control of very large scale systems. With the circuitry on cards that are attached to a standardized bus, the rack mount system offers greater networking and communication possibilities. This allows PLCs to run lengthy programs and control complex automated systems.
In its most basic configuration the PLC reads a set of inputs, processes the inputs through a program, and makes a decision as to what output switching is required. The inputs are configured to sense the presence of a voltage, often 12VDC, but may be 5VDC or 115VAC. These inputs come from switches or relays on machines that may indicate level, position, or status of the equipment. A PLC can be used to stop a conveyor belt when an assembly machine is turned off. Sensors are widely used in process control to detect