GE PACSystems RX7i

Chapter 8. Structured Text (ST) Programming
GFK-2950C February 2018 379
8.2.7 WHILE Statement
The WHILE loop repeatedly executes (iterates) a statement list contained within the
WHILE…END_WHILE construct as long as a specified condition is TRUE (1). It checks the condition
first, then conditionally executes the statement list. This looping construct is useful when the
statement list does not necessarily need to be executed.
Format
WHILE <BooleanExpression> DO
<StatementList>;
END_WHILE;
Where:
BooleanExpression Any expression that resolves to a Boolean value.
StatementList Any set of Structured Text statements.
Operation
If BooleanExpression is FALSE (0), the loop is immediately exited; otherwise, if the
BooleanExpression is TRUE (1), the StatementList is executed and the loop repeated. The statement
list may never execute, since the Boolean expression is evaluated at the beginning of the loop.
Note: It is possible to create an infinite loop that will cause the watchdog timer to expire. Avoid
infinite loops.
Example
The following code fragment increments J by a value of 2 as long as J is less than or equal to 100.
WHILE J <= 100 DO
J := J + 2;
END_WHILE;