[Bug] All down-counters (CTD/CTUD + typed variants) count on FALLING edge of CD, violating IEC 61131-3

[chiouiss]

Summary

In the standard function block library (libs/sources/iec-standard-fb/counter.st, current main; also shipped with OpenPLC Editor 4.2.11 under resources/strucpp/libs/sources/), every down-counting input CD is edge-detected with F_TRIG (falling edge), while every up-counting input CU correctly uses R_TRIG (rising edge) — 10 occurrences of CD_T : F_TRIG on main as of 2026-08-23.

IEC 61131-3 defines both CU and CD as rising-edge-sensitive inputs. As implemented, a down-count is registered one pulse later than expected — when the input pulse ends rather than when it begins.

Affected blocks (10 total)

All down-counters in counter.st share the pattern:

• CTD, CTD_DINT, CTD_LINT, CTD_UDINT, CTD_ULINT
• CTUD, CTUD_DINT, CTUD_LINT, CTUD_UDINT, CTUD_ULINT

All CTU* blocks are correct (R_TRIG).

Evidence (source excerpts)

CTUD — note the asymmetry between CU and CD:

VAR
    CU_T : R_TRIG;   (* up-count: rising edge - IEC-compliant *)
    CD_T : F_TRIG;   (* down-count: FALLING edge - non-compliant *)
END_VAR
CU_T(CLK := CU);
CD_T(CLK := CD);

CTD — same pattern:

VAR
    CD_T : F_TRIG;
END_VAR
CD_T(CLK := CD);
IF LD THEN
    CV := PV;
ELSIF CD_T.Q AND (CV > 0) THEN
    CV := CV - 1;
END_IF;

Steps to reproduce

Target: OpenPLC Simulator (Editor 4.2.11), cyclic task T#20ms, Structured Text.

PROGRAM repro
  VAR
    cnt : CTUD;
    pulse : BOOL;
  END_VAR
cnt(CU := FALSE, CD := pulse, R := FALSE, LD := FALSE, PV := 10);
END_PROGRAM

1. Preload CV to a non-zero value (count up twice via CU, or use CTD + LD).
2. Force pulse from FALSE to TRUE. Expected (IEC): CV decrements on this scan. Actual: CV unchanged.
3. Force pulse back to FALSE. Actual: CV decrements now — i.e. on the falling edge.

The asymmetry is directly observable with CTUD: wire the same pulse pattern to CU and CD alternately; up-counts land on pulse start, down-counts land on pulse end.

Expected behavior

Per IEC 61131-3 (standard counter FBs), CD shall be sampled on its rising edge, symmetric with CU.

Suggested fix

Replace F_TRIG with R_TRIG for every CD_T declaration in counter.st (10 occurrences). No interface change; only the edge timing of down-counts is corrected.

Note: this changes observable timing for any user program that (knowingly or not) relies on the current falling-edge behavior, so it may warrant a release-note entry.

Environment

• OpenPLC Editor 4.2.11 (Windows x64, installer OpenPLC.Editor_4.2.11.exe)
• Library file: libs/sources/iec-standard-fb/counter.st (verified identical behavior in the bundled copy and current main)
• Reproduced on the built-in OpenPLC Simulator target