Rockwell Automation Logix5000 Controllers Structured Text Programming Manual - Manual (Page 31)

Publication 1756-PM007D-EN-P - November 2012 31
Program Structured Text Chapter 1
WHILE…DO Use the WHILE…DO loop to keep doing something as long as certain
conditions are true.
Operands:
Structured Text
Description: The syntax is:
Operand Type Format Enter
bool_
expression BOOL Tag
Expression
BOOL tag or expression that evaluates to
a BOOL value
WHILE bool_expression DO
<statement>;
END_WHILE;
IMPORTANT
Make sure that you do not iterate within the loop too many times in a single
scan.
• The controller does not execute any other statements in the routine until
it completes the loop.
• If the time that it takes to complete the loop is greater than the
watchdog timer for the task, a major fault occurs.
• Consider using a different construct, such as IF...THEN.
WHILE bool_expression1 DO
<statement>; Statements to execute while
bool_expression1 is true
Optional
IF bool_expression2 THEN
EXIT; If there are conditions when you want to
exit the loop early, use other statements,
such as an IF...THEN construct, to
condition an EXIT statement.
END_IF;
END_WHILE;