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Programmable Logic Controller  (PLC)

A programmable logic controller, PLC, or programmable controller is a digital computer used for automation of typically industrial electromechanical processes, such as control of machinery on factory assembly lines. PLCs can be found in nearly all new machinery control systems and industrial processes. PLCs are designed for multiple arrangements of digital and analog inputs and outputs. Programs to control machine operation are typically stored in battery- backed-up or non-volatile memory. A PLC is an example of a "hard" real-time system since output results must be produced in response to input conditions within a limited time, otherwise unintended operation will result. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was mainly composed of relays, cam timers, drum sequencers, and dedicated closed-loop controllers. Since these could number in the hundreds or even thousands, the process for updating such facilities for the yearly model change-over was very time consuming and expensive, as electricians needed to individually rewire the relays to change their operational characteristics.  What Is Inside A PLC?  The Central Processing Unit, the CPU, contains an internal program that tells the PLC how to perform the following functions: Execute the Control Instructions contained in the User's Programs. Communicate with other devices, which can include I/O Devices, Programming Devices, Networks, and even other PLCs. Perform Housekeeping activities such as Communications, Internal Diagnostics, etc. How Does A PLC Operate? There are four basic steps in the operation of all PLCs; Input Scan, Program Scan, Output Scan, and Housekeeping. These steps continually take place in a repeating loop. Four Steps In The PLC Operations 1.) Input Scan Detects the state of all input devices that are connected to the PLC 2.) Program Scan Executes the user created program logic 3.) Output Scan Energizes or de-energize all output devices that are connected to the PLC 4.) Housekeeping This step includes communications with programming terminals, internal diagnostics, etc... These steps are continually processed in a loop. What Programming Language Is Used To Program A PLC? While Ladder Logic is the most commonly used PLC programming language, it is not the only one. Other programming languages include:   - Function Block Diagram (FBD) - A graphical language for depicting signal and data flows through re-usable function blocks. FBD is very useful for expressing the interconnection of control system algorithms and logic. - Structured Text (ST) – A high level text language that encourages structured programming. It has a language structure (syntax) that strongly resembles PASCAL and supports a wide range of standard functions and operators. - Instruction List (IL): A low level “assembler like” language that is based on similar instructions list languages found in a wide range of today’s PLCs.   - Sequential Function Chart (SFC). A method of programming complex control systems at a more highly structured level. A SFC program is an overview of the control system, in which the basic building blocks are entire program files. Each program file is created using one of the other types of programming languages. The SFC approach coordinates large, complicated programming tasks into smaller, more manageable tasks.