This is Control Tech's online glossary of commonly-used automation terms, acronyms, and buzzwords. If you don't find it here, send email to techpubs@ctc-control.com and we'll put it in.

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I/O

Stands for "Input/Output," in automation, a term encompassing all physical connection points between a control system and the sensors and actuators of the "real world."

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PLC

Stands for "Programmable Logic Controller", a microprocessor-based system used for controlling industrial machinery. These typically accept plug-in Input/Output modules for making connection to sensors and actuators. Most PLCs are programmed using a visual programming technique called "Relay Ladder Logic", developed in the 1960s.

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Relay Ladder Logic

This programming language expresses a program as a series of "coils" and "contacts", simulating the operation of electromechanical relays. The resultant program is the equivalent of a boolean equation, which is executed continuously in a combinatorial manner. The advantage of this language is the familiarity many electricians have with the simple operation of relays. Disadvantages include the complexity of large, cross-connected programs, and the difficulty of expressing such non-binary functions as motion control and analog I/O. An alternative technique now being widely used is State Language control.

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State Languages

State languages are used primarily for expressing sequences of operations in the real world. Typically, these languages divide a program into a series of "states," or steps - the program executes wholly in one step until some transition event occurs which causes the program to move to a new step. As such, it represents a reasonable way to express the operation of machinery, which itself must transit through a series of mechanical states as it operates. Practical state languages used for automation must allow multitasking, whereby multiple threads of steps may be in operation at once.

For more information about State Language, see the white paper, State Language for Machine Control, by Kenneth Crater (also available as a PDF file).