CTUD — Count Up / Down

Structured Text up/down counter function block. Uses FBD_COUNTER; combines the roles of ladder CTU and CTD.

Operands

Name	Type
`counter`	`FBD_COUNTER`

Member	Type	Meaning
`.EnableIn`	`BOOL`	`1` lets the instruction run; `0` holds all outputs unchanged. Normally `1`.
`.CUEnable`	`BOOL`	Rising edge (`0` to `1`) increments `ACC` by `1`
`.CDEnable`	`BOOL`	Rising edge (`0` to `1`) decrements `ACC` by `1`
`.PRE`	`DINT`	Preset target count
`.Reset`	`BOOL`	`1` clears `CU`, `CD`, `DN`, `OV`, `UN` and sets `ACC` to `0`
`.EnableOut`	`BOOL`	`1` while the instruction is executing normally
`.ACC`	`DINT`	Accumulated count
`.CU`	`BOOL`	`1` while `CUEnable` is `1` and `Reset` is `0`
`.CD`	`BOOL`	`1` while `CDEnable` is `1` and `Reset` is `0`
`.DN`	`BOOL`	`1` when `ACC` is greater than or equal to `PRE`
`.OV`	`BOOL`	`1` when `ACC` overflows past `2147483647` and wraps to `-2147483648`
`.UN`	`BOOL`	`1` when `ACC` underflows past `-2147483648` and wraps to `2147483647`

CTUD is the Structured Text function-block counter. It combines the roles of ladder CTU and CTD on a single FBD_COUNTER tag.

CarsInLot.CUEnable := EntrySensor;
CarsInLot.CDEnable := ExitSensor;
CarsInLot.PRE := 20;
CTUD(CarsInLot);

LotFull := CarsInLot.DN;

Setting .EnableIn := 0 and expecting the counter to stop. EnableIn = 0 freezes all outputs, including .ACC, .DN, and the status bits — the counter goes dormant. To stop counting, hold .CUEnable and .CDEnable at 0 instead and leave .EnableIn at 1.