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Programmable Logic Controllers, 5th Edition by William Bolton

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Chapter 1. Programmable Logic Controllers
This chapter is an introduction to the programmable logic controller (PLC) and its general function, hardware forms, and internal architecture. This overview is followed by more detailed discussion in the following chapters.

1.1. Controllers

What type of task might a control system handle? It might be required to control a sequence of events, maintain some variable constant, or follow some prescribed change. For example, the control system for an automatic drilling machine (Figure 1.1a) might be required to start lowering the drill when the workpiece is in position, start drilling when the drill reaches the workpiece, stop drilling when the drill has produced the required depth of hole, retract the drill, ...

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