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PLCs were originally designed to replace relays, and very simple PLCs may be referred to as programmable logic relays (PLRs). FLIP PDF PRO 2.4.1 LOGO REMOVAL CODEPLCs also differ from desktop computers in that they do not typically use a commercially available operating system (OS) instead they rely on blocks of logic code that allow the PLC to function automatically to specific inputs (e.g., from sensors) with as little overhead as possible. Unlike desktop computers, PLCs are typically materially hardened (making them suitable for deployment on a production floor) and may be specialized for specific industrial uses with multiple specialized inputs and outputs. However, implementation of safety interlocks in programmable electronic systems is not universally accepted and many organizations require that all safety interlocks be hard-wired.Įric Knapp, in Industrial Network Security, 2011 PLCsĪ programmable logic controller (PLC) is a specialized computer used to automate functions within industrial networks. Programmable logic controllers are a potential candidate for implementing interlocks due to excellent capability of implementing discrete logic. Therefore, for applications that require extensive use of discrete logic, most DCS suppliers will incorporate PLCs into their system. The process control modules of a DCS often implement discrete logic, but their discrete I/O capabilities and slow scan rates rarely match that of a PLC. Scan rates much faster than 100 msec are common. The manufacturers of PLCs provide robust, cost-effective discrete I/O modules that can scan discrete data points (sensors) very rapidly. They can be programmed using the same ladder logic diagrams or other textual and graphical programming languages to represent logic and are an excellent tool to implement discrete logic. Programmable logic controllers (PLCs) were developed in the 1970s in order to replace hard-wired relay logic. Edgar, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 VII.C Programmable Logic Controllers
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