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Structured text, abbreviated as ST or STX, is one of the five languages supported by the IEC 61131-3 standard, designed for programmable logic controllers (PLCs). [ 1 ] [ 2 ] It is a high level language that is block structured and syntactically resembles Pascal , on which it is based. [ 3 ]
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
ST (structured text) is similar to programming in Pascal or C; 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
As programming terminals evolved, because ladder logic was a familiar format used for electro-mechanical control panels, it became more commonly used. Newer formats, such as state logic, [18] function block diagrams, and structured text exist. Ladder logic remains popular because PLCs solve the logic in a predictable and repeating sequence, and ...
Many vendors whilst incorporating the full IEC 61131-3 requirements have additional vendor specific calls/function blocks to suit their hardware such as reading or writing to I/O. Siemens PLC instruction list language is known as "Statement List" or "STL" in English, and "Anweisungs-Liste" or "AWL" in German, Italian and Spanish.
They employ standardized control programming languages (such as those under IEC 61131-3, a suite of five programming languages including function block, ladder, structured text, sequence function charts and instruction list), that are frequently used to create programs which run on these RTUs and PLCs.
The IEC 61131-3 is a standard defining the programming languages for PLCs, embedded controls, and industrial PCs, harmonizing applications independent from specific dialects, but still based on known methods such as the textual programming languages Instruction List, and Structured Text, the graphical programming languages Function Block ...
Note: In this example, "Run" represents the status of a bit in the PLC, while "Motor" represents the actual output to the real-world relay that closes the motor's real-world circuit. For safety reasons, an emergency stop ("ES") may be hardwired in series with the "Start" switch, and the relay logic should reflect this.