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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 may result. Programs to control machine operation are typically stored in battery-backed-up or non-volatile memory .
IEC 61131-3 is the third part (of 10) of the international standard IEC 61131 for programmable logic controllers.It was first published in December 1993 [1] by the IEC; the current (third) edition was published in February 2013.
They include timers, arithmetic operators and comparisons, table lookups, text processing, PID control, and filtering functions. More powerful PLCs can operate on a group of internal memory locations and execute an operation on a range of addresses, for example, to simulate a physical sequential drum controller or a finite-state machine. In ...
For example, a watchdog timer may be used when running untrusted code in a sandbox, to limit the CPU time available to the code and thus prevent some types of denial-of-service attacks. [2] In real-time operating systems , a watchdog timer may be used to monitor a time-critical task to ensure it completes within its maximum allotted time and ...
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.
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]
Depending on the device and its connection modules, signals may be a simple binary value ("high" or "low") or more complex. More complex inputs, outputs, and calculations were also supported as the SIMATIC line developed. For example, the SIMATIC 505 could handle floating point quantities and trigonometric functions. [2]
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.
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