CTD — Counter-Down

Pin layout

Direction	Name	Type	Description
Input	`CD`	`BOOL`	Count Down — decrement CV on each rising edge
Input	`LD`	`BOOL`	Load — when TRUE, set CV := PV
Input	`PV`	`INT`	Preset Value loaded by LD
Output	`Q`	`BOOL`	TRUE when CV <= 0
Output	`CV`	`INT`	Current Value

Semantics

LD = TRUE  : CV := PV
CD rising  : CV := CV - 1 (saturates at INT min)
Q := CV <= 0

LD has priority over CD. Use LD to (re-)initialise the counter to its preset.

Example

VAR
    countdown : CTD;
END_VAR

countdown(CD := xPulse, LD := xReset, PV := 10);

IF countdown.Q THEN
    (* zero reached *)
    DoFinish();
END_IF;

IEC reference

IEC 61131-3 third edition (2013), Annex F.6.2.

matiec conformance

Implemented per the standard. Same edge-detection caveat as CTU: CD must see a real rising edge per increment.