Iteration (FOR / WHILE / REPEAT)

Iteration

Three loop constructs in IEC 61131-3 ST:

• FOR — counter-driven. Iterate a known number of times with a controlled loop variable. Use when the iteration count is computable upfront.
• WHILE — pre-test condition. Loop body may run zero times if the condition is FALSE on entry.
• REPEAT — post-test condition. Loop body always runs at least once.

Plus two flow-control statements that work inside any loop:

• EXIT — terminates the innermost enclosing loop immediately.
• CONTINUE — skips to the next iteration of the innermost enclosing loop.

Loops on a PLC: scan-cycle hazard

Loops in PLC code have a special concern that desktop programmers don’t worry about: a long-running loop blocks the scan cycle. If your WHILE runs for 50 ms in a 20 ms task, you’ve already missed a deadline before the next scan even starts. Outputs don’t update; live monitoring stops; the PLC may be flagged as overrun by the runtime.

Two consequences:

1. Bound every loop. A WHILE x DO ... END_WHILE without a counted exit clause is a code smell on a PLC. Add a safety-counter:

   iSafetyCount := 0;
   WHILE NOT xDone AND iSafetyCount < 100 DO
       (* work *)
       iSafetyCount := iSafetyCount + 1;
   END_WHILE;
   

2. Prefer step-machines over long loops. If the work would take many iterations, model it as an SFC (or a hand-rolled step counter) and do one iteration per scan. The PLC is a cyclic executor — work with the cycle, not against it.

EXIT and CONTINUE

FOR i := 1 TO 100 DO
    IF arr[i] = TARGET THEN
        iFound := i;
        EXIT;            (* leave the loop early *)
    END_IF;
END_FOR;

FOR i := 1 TO 100 DO
    IF arr[i] = 0 THEN
        CONTINUE;        (* skip rest of body, advance i *)
    END_IF;
    iSum := iSum + arr[i];
END_FOR;

EXIT only leaves the innermost enclosing loop; nested loops need an EXIT per level (or a flag-and-break pattern). There is no EXIT 2 form in IEC 61131-3.

IEC reference

• FOR: clause 6.6.3.4.
• WHILE: clause 6.6.3.5.
• REPEAT: clause 6.6.3.6.
• EXIT: clause 6.6.3.7.

matiec conformance

matiec implements all three loop constructs and EXIT / CONTINUE per the standard.