ABSTRACT
Since a programmable logic controller (PLC) is basically a digital device that monitors, processes, and generates data from an industrial process, it requires a software program with instructions to perform the desired functions. This program has two separate parts with two different objectives. PLC programming included only digital input and output instructions that performed only logical functions corresponding to the case of conventional electromechanical relays. Until about 1990, PLC manufacturers were developing their proprietary programming languages with complete incompatibility between them. As an example, ladder languages from two different manufacturers may have had a similar form, but under no circumstances were they compatible. The International Electrotechnical Commission (IEC) 61131-3 standard incorporates five different approaches to programming control systems, and especially for the case of PLCs. This chapter presents a sequential function chart (SFC) program for a two-door security system where one door requires a three-digit security code and the other one requires a two-digit security code to open.
