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plc st -19.2

PROGRAM main VAR

i: INT; END_VAR i:= 0; REPEAT

i :=i +1;

UNTIL i >= 10; END_REPEAT; END_PROGRAM

Figure 19.1 A Structured Text Example Program

One important difference between ST and traditional programming languages is the nature of program flow control. A ST program will be run from beginning to end many times each second. A traditional program should not reach the end until it is completely finished. In the previous example the loop could lead to a program that (with some modification) might go into an infinite loop. If this were to happen during a control application the controller would stop responding, the process might become dangerous, and the controller watchdog timer would force a fault.

ST has been designed to work with the other PLC programming languages. For example, a ladder logic program can call a structured text subroutine. At the time of writing, Allen Bradley offers limited support for ST programming, but they will expand their support in the future.

19.2 THE LANGUAGE

The language is composed of written statements separated by semicolons. The statements use predefined statements and program subroutines to change variables.

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