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Keywords: PLC, programmable logic controller, industrial controls, fieldbus, analog input, analog output,
Profibus, factory automation, electronic components
TUTORIAL 4701
Introduction to Programmable Logic Controllers
(PLCs) and the Operational Function of Main
System Modules
By: Hugh Wright
May 10, 2010
Abstract: This tutorial offers an in-depth introduction into programmable logic controllers (PLCs). The
article starts with an overview of the history and the role PLCs in factory automation. The basic principles
of PLC operation are discussed. The core modules of an industrial-control system are examined: the
analog input function, analog output functions, the distributed control (fieldbus) interface, digital inputs
and outputs (I/Os), the CPU, and isolated power. Functional block diagrams and recommended devices
are included for each function.
Overview
Programmable logic controllers (PLCs) have been an integral part of factory automation and industrial
process control for decades. PLCs control a wide array of applications from simple lighting functions to
environmental systems to chemical processing plants. These systems perform many functions, providing
a variety of analog and digital input and output interfaces; signal processing; data conversion; and
various communication protocols. All of the PLC's components and functions are centered around the
controller, which is programmed for a specific task.
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