CTUD — Counter Up/Down

Pin layout

Direction	Name	Type	Description
Input	`CU`	`BOOL`	Count Up — increment on rising edge
Input	`CD`	`BOOL`	Count Down — decrement on rising edge
Input	`R`	`BOOL`	Reset — when TRUE, CV := 0 (priority over LD)
Input	`LD`	`BOOL`	Load — when TRUE, CV := PV
Input	`PV`	`INT`	Preset Value used by both LD and QU
Output	`QU`	`BOOL`	TRUE when CV >= PV
Output	`QD`	`BOOL`	TRUE when CV <= 0
Output	`CV`	`INT`	Current Value

Semantics

R = TRUE              : CV := 0    (priority)
LD = TRUE             : CV := PV
CU rising and not R/LD: CV := CV + 1
CD rising and not R/LD: CV := CV - 1
QU := CV >= PV
QD := CV <= 0

If CU and CD both rise on the same scan, matiec applies CU first then CD — net effect is no change. Don’t rely on this ordering across other implementations.

Example

VAR
    netCount : CTUD;
END_VAR

netCount(CU := xUp,
         CD := xDown,
         R  := xReset,
         LD := xLoad,
         PV := 100);

iCurrent := netCount.CV;
xAtTop   := netCount.QU;
xAtBottom := netCount.QD;

IEC reference

IEC 61131-3 third edition (2013), Annex F.6.3.

matiec conformance

Implemented per the standard.