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`	Forced to `1` each scan by the instruction call — setting it `0` has no effect
`.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.

Each rising edge on .CUEnable increments .ACC by one.
Each rising edge on .CDEnable decrements .ACC by one.
.DN is 1 when .ACC is greater than or equal to .PRE.
.Reset clears .ACC and status bits.
An enable that is already 1 when the program starts does not count. The counter only counts a 0 → 1 change it sees while the program is running. (Prescan sets this up before the first scan.)

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

LotFull := CarsInLot.DN;

Practice

Try CTUD in the Count Up Down exercise — track a running balance with separate Up and Down inputs.

Trying to stop the counter with .EnableIn := 0. The instruction call forces .EnableIn back to 1 every scan, so the assignment has no effect. To stop counting, hold .CUEnable and .CDEnable at 0 (no rising edges).
Assigning .ACC from logic to reset it (MyCounter.ACC := 0). Structured Text recomputes .ACC every scan, so a runtime write is ignored (the initial .ACC default is still honored). Clear it with .Reset := 1. The same write does take effect on the ladder CTU / CTD — so don't rely on it either way; it isn't portable.