ISPSoft User Manual

Chapter 11 Structured Text
A complete statement corresponds to a network in a ladder diagram. It represents a task. Take the
sub-statement shown above for example. The value of the arithmetic expression D0* (D1+D2) is
calculated, and the result of the operation is stored in D10. The arithmetic expression in the red
frame in the figure below is not a legal statement. It is an expression. It is not a concrete task. It is
like the ladder diagram shown below. The ladder diagram lacks an output operand. It is not a
complete program which can be executed.
A structured text is composed of statements, and a statement is composed of elements. An example
of a structured text is shown below. The complete statements in the structured text are in the red
frames.
* In a structured text, “:=” means “to assign”, and “=” means “to be equal to”.
11.1.3 Expression
An expression is an important element of a statement. It represents a value. An expression can be
an arithmetic expression, a constant, a symbol, or a device. Examples are listed below.
• M0 & M1 (Boolean value)
The logical operator & takes M0 and M1, and performs the logical & operation on M0 and M1.
The result of the operation is a Boolean value.
• M0 = FALSE (Boolean value)
The expression is a conditional expression. If M0 is ON, the Boolean value is TRUE. If M0 is OFF,
the Boolean value is TRUE.
• M0 (Boolean value)
If M0 is ON, the Boolean value is TRUE. If M0 is OFF, the Boolean value is FALSE.
• D1 + D2 (Value)
The value of the arithmetic expression D1+D2 is calculated.
• D0 (Value)
The value represented by the expression if the value stored in D0.
• D2 = D0 + D1 (Boolean value)
If the value of the arithmetic expression D0+D1 is equal to the value stored in D2, the Boolean
value is TRUE. If the value of the arithmetic expression D0+D1 is not equal to the value stored in
D2, the Boolean value is FALSE.
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