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Welcome to Module 24, which covers Programmable Logic Controllers , or PLCs. The Programmable Logic Controller (PLC) was invented in the 1960s to replace the sequential Relay Circuits traditionally used in machine control. A PLC is a solid-state, electronic device that controls the operation of a machine. It uses Logic functions, which are programmed into its memory via programming software. Almost any “real world” application that needs electrical control needs a PLC. In fact, whether you work in machining, packaging, material handling, automated assembly, or countless other industries, you are probably already using a PLC.
Figure 1. A Programmable Logic Controller Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully, then answer the questions without referring back to what you’ve just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation to build on as you move from topic to topic and module to module. A Note on Font Styles Key points are in bold. Glossary items are italicized and underlined the first time they appear.
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