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WHILE loop

Syntax

WHILE <bool_expr> DO
    <statement_list>
END_WHILE;

Semantics

  1. Evaluate <bool_expr>. If FALSE, skip the body and continue after END_WHILE.
  2. Run the body.
  3. Go to step 1.

The body may run zero times if the condition is FALSE on entry. This is the difference from REPEAT.

Examples

(* Drain a queue until empty *)
WHILE iQueueLen > 0 DO
    ProcessOne();
    iQueueLen := iQueueLen - 1;
END_WHILE;

(* Bounded poll *)
iSafetyCount := 0;
WHILE NOT xReady AND iSafetyCount < 50 DO
    DoOneScan();
    iSafetyCount := iSafetyCount + 1;
END_WHILE;

Scan-cycle hazard

WHILE without an explicit per-iteration safety counter is a red flag in PLC code. Even if the body looks bounded, a runtime data condition (sensor stuck, queue mis-counting) can keep the loop running until the scan cycle is missed.

Always add an iSafetyCount and an upper bound that fits into your task’s CPU budget.

IEC reference

IEC 61131-3 third edition (2013), clause 6.6.3.5.

matiec conformance

Implemented per the standard. The strict BOOL-condition rule is the main porting pain point from C/Pascal where any non-zero value is treated as truthy.