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Chapter 8 Structured Text (ST) Programming
The Structured Text (ST) programming language is an IEC 61131-3 textual programming language.
This chapter describes how structured text is implemented in PACSystems. For information on using
the structured text editor in the programming software, refer to the online help.
The block types Block, Parameterized Block, and Function Block (UDFB) can be programmed in ST.
The _MAIN program block can also be programmed in ST. For details on blocks, refer to Program
Organization in Chapter 2.
8.1 Language Overview
8.1.1
Statements
A structured text program consists of a series of statements, which are constructed from
expressions and language keywords. A statement directs the PACSystems controller to perform a
specified action. Statements provide variable assignments, conditional evaluations, iteration, and the
ability to call built-in functions. PACSystems supports those statements described in Statement
Types.
8.1.2
Expressions
Expressions use operators to calculate values from variables and constants. An example of a simple
expression is (x + 5).
Composite expressions can be created by nesting simpler expressions, for example,
(a + b) * (c + d) – 3.0 ** 4.
GFK-2950C
February 2018
365
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