Relays
The Programmable Logic Controller
Early machines were controlled by mechanical means using cams, gears, levers and other basic mechanical devices. As the complexity grew, so
did the need for a more sophisticated control system. This system contained wired relay and switch control elements. These elements were wired as required to provide the control logic necessary for the particular type of machine operation. This was acceptable for a machine that never needed to be changed or modified, but as manufacturing techniques improved and plant changeover to new products became more desirable and necessary, a more versatile means of controlling this equipment had to be developed. Hardwired relay and switch logic was cumbersome and time consuming to modify. Wiring had to be removed and replaced to provide for the new control scheme required. This modification was difficult and time consuming to design and install and any small "bug" in the design could be a major problem to correct since that also required rewiring of the system. A new means to modify co
ntrol circuitry was needed. The development and testing ground for this new means was the . auto industry. The time period was the late 1960's and early 1970's and the result was the programmable logic controller, or PLC. Automotive plants were confronted with a change in manufacturing techniques every time a model changed and, in some cases,  for changes on the same model if improvements had to be made during the model year.  The PLC provided an easy way to reprogram the wiring rather than actually rewiring the control system.
The PLC that was developed during this time was not very easy to program. The language was cumbersome to write and required highly trained programmers. These early devices were merely relay replacements and could do very little else. The PLC has at first gradually, and in recent years rapidly developed into a sophisticated and highly versatile control system component. Units today are capable of performing complex math functions including numerical integration and differentiation and operate at the fast microprocessor speeds now available. Older PLCs were capable of only handling discrete inputs and outputs that is, on-off type signals, while today's systems can accept and generate analog
voltages
and currents as well as a wide range of voltage levels and pulsed signals. PLCs are also designed to be rugged. Unlike their personal computer cousin, they can typically withstand vibration, shock, elevated temperatures, and electrical noise to which manufacturing equipment is exposed.
As more manufacturers become involved in PLC production and development, and PLC capabilities expand, the programming language is also expanding. This is necessary to allow the programming of these advanced capabilities. Also, manufacturers tend to develop their own versions of ladder logic language the language used to program PLCs.  This complicates learning to program PLC's in general since one language cannot be learned that is applicable to all types. However, as with other computer languages, once the basics of PLC operation and programming in ladder logic are learned, adapting to the various manufacturers’ devices is not a complicated process. Most system designers eventually settle on one particular manufacturer that produces a PLC that is personally comfortable to program and has the capabilities suited to his or her area of applications.
It should be noted that in usage, a programmable logic controller is generally referred to as a “PLC” or “programmable controller”. Although the term “programmable controller” is generally accepted, it is not abbreviated “PC” because the abbreviation “PC”  is usually used in reference to a personal computer. As we will see in this chapter, a PLC is by no means a personal computer.controller翻译中文
Programmable controllers the shortened name used for programmable logic controllers are much like personal computers in that the user can be overwhelmed by the vast array of options and configurations available. Also, like personal computers, the best teacher of which one to select is experience. As one gains experience with the various options and configurations available, it becomes less confusing to be able to select the unit that will best perform in a particular application.
The typical system components for a modularized PLC are:
1. Processor.
The processor sometimes call a CPU, as in the self contained units, is generally specified according to memory required for the program to  be In the rmodularizeversions, capability can also be a factor.  This includes features such as highe math functions, PID control loops and  optional programming commands. The processor consists of the microprocessor, system memory, serial communication ports for printer,  PLC LAN link and external programming device and, in some cases, the system power supply to power the processor and I/O modules.
2. Mounting rack.
This is usually a metal framework with a printed circuit board backplane which provides means for mounting the PLC input/output I/O  modules and processor. Mounting racks are specified according to the number of modules required to implement the system. The mounting rack provides data and power connections to the processor and modules via the backplane. For CPUs that do not contain a power supply, the rack also holds the modular power supply. There are systems in which the processor is mounted separately and connec
ted by cable to the rack. The mounting rack can be available to mount directly to a panel or can be installed in a standard 19" wide equipment cabinet. Mounting racks are cascadable so several may be interconnected to allow a system to accommodate a large number of I/O modules.

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