CNTDN - Down Counter

Topic: DMD0031

Help File Version: 2.9.4.37

CNTDN - Down Counter

The Down Counter (CNTDN) instruction is used to accumulate OFF to ON state changes of input logic, counting downward from a preset value toward 0. This instruction has the following two ladder logic inputs:

The first input the Down Count input (DN). Each time this input logic transitions from OFF to ON the Counter's accumulator (.Acc) is decremented by one. The Counter's .Done bit will turn ON when the .Acc value reaches 0.

The second input is the Count Reset (RST). When this input logic in ON, the Down Counter will reset and the Counter's accumulator will return to the Preset Value. The Counter will remain in the reset state as long as the Reset input remains ON. The Reset input has priority over the Down Count input, meaning that if the Down Count input is ON at the same time as the Reset input, the Counter will not count.

Parameters:

Note: Use the F9 key to open the Default Element Selection Tool (the Element Picker or the Element Browser) or use the Down-Arrow key (Auto-Complete) on any parameter field to see a complete list of the memory locations that are valid for that parameter of the instruction.

Counter Struct specifies which Counter number to edit, and by extension, the name of the Counter's associated structure. This can be any of the System-created Counters, or any user-created Counter. The default Memory Configuration contain 255 predefined Counters named CT0 through CT255.

Preset is the count value at which the Counter starts counting. Each time the Counter is reset the Counter's accumulator is initialized with this Preset value. This can be any positive 32-bit constant value (0 to 2,147, 483, 647) or any readable numeric location containing a value in that range.

Termination Scan Behavior:

All Counters - with one exception - have termination logic. This means if the Counter instruction is contained within a Program, a Task, or a Stage , the Counter will automatically be reset any time the containing Program, Task, or Stage stops executing because of an End Program, End Task, or Disable Stage respectively.

The Global Up / Down Counter (UDCG) instruction does NOT have termination logic, meaning that it will retain it's state information even if it is contained within a Program, a Task, or a Stage, that is disabled.

Refer to the Help topic on Termination Behavior for detailed information on the programming elements that have termination logic.

Status Display:

The yellow triangle in the upper left corner indicates this is a Multi-Scan instruction.

The gray triangle to the right of the DN Count input leg indicates this input is edge-triggered, meaning that each time the input logic transitions from OFF to ON the Counter's accumulator will decrement by one count.

Counter Structure Members

All Counters have an associated structure with fields that can be used in the ladder program. The structure fields are updated whenever the Counter instruction is processed in the PLC scan. The syntax for using them is <Counter structure>.<field name>, for example: CT1.Acc.

.Acc (read/write) is a 32-bit integer value that represents the current number of counts that have accrued.

.Done (read-only) when counting upward toward a preset value, this Bit will be ON any time the value in the Counter's accumulator (.Acc) is greater than or equal to the Counter's Preset value.

.Reset (read-only) will be ON if the RESET logic is ON, or if the Counter is being held in reset by a Reset Counter (RSTCT) instruction. The Reset input has priority over the UP input, meaning that if the UP input is ON at the same time as the Reset input, the Counter will not count.

.Zero (read-only) will be ON any time the value in the Counter's accumulator value (.Acc) is 0.

.DnDone (read-only) when counting downward toward the down preset value, this Bit will ON any time the value in the Counter's accumulator value (.Acc) is less than or equal to the Counter's Preset value. This bit field is only used by the Up / Down Counter (UDC) and the Global Up / Down Counter (UDCG) .