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Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:   (doi:10.13031/2013.17770) in Programmable Logic Controllers. Chapter 10 in Fundamentals of Electricity for Agriculture, 3rd edition, 277-300. St. Joseph, Michigan: ASAE. . Copyright 2004 American Society of Agricultural and Biological Engineers, St. Joseph, Mich.
Authors:   Gustafson, Robert J., and Mark T. Morgan
Keywords:   PLC Construction, PLC Operation, I/O Module Details and Connections, Operating Modes, PLC Programming, PLC Addressing, PLC Instructions, IEC 61131-3 PLC Programming Standard

This chapter will cover the basics of programmable logic controllers (PLCs). PLCs can perform all of the functions of relay-based control systems and much more. They have replaced hard-wired relay-based control systems in most industries because of their compact size, ruggedness, and most importantly, their ability to be easily reprogrammed. Changes in a control system or process function can be made with minimal re-wiring.

One reason for their rapid adoption was that they could be programmed using the same language used for relays, i.e., ladder logic diagrams. For this same reason, understanding how PLCs operate is generally only a small step from understanding ladder logic diagrams.

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