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A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity that requires high reliability, ease of programming, and process fault diagnosis.
Ladder diagrams were once the only way to record programmable controller programs, but today, other forms are standardized in IEC 61131-3. For example, instead of the graphical ladder logic form, there is a language called Structured text , which is similar to C, within the IEC 61131-3 standard.
Function Block Diagram is one of five languages for logic or control configuration [2] supported by standard IEC 61131-3 for a control system such as a programmable logic controller (PLC) or a Distributed Control System (DCS). The other supported languages are ladder logic, sequential function chart, structured text, and instruction list.
Part 3 of IEC 61131 deals with basic software architecture and programming languages of the control program within PLC. It defines three graphical and two textual programming language standards: Ladder diagram (LD), graphical; Function block diagram (FBD), graphical; Structured text (ST), textual
LD (ladder diagram) enables programmers to virtually combine relay contacts and coils; FBD (function block diagram) enables users to rapidly program both Boolean and analog expressions; SFC (sequential function chart) is convenient for programming sequential processes and flows; Additional graphical editor available in CODESYS:
Early examples of FPGAs are the 82S100 array, and 82S105 sequencer, by Signetics, introduced in the late 1970s. The 82S100 was an array of AND terms. The 82S105 also had flip-flop functions. (Remark: 82S100 and similar ICs from Signetics have PLA structure, AND-plane + OR-plane.)
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A feedback control loop is directly controlled by the RTU or PLC, but the SCADA software monitors the overall performance of the loop. For example, a PLC may control the flow of cooling water through part of an industrial process to a set point level, but the SCADA system software will allow operators to change the set points for the flow.