The Down Count (CTD) instruction counts down from the current value each time the input energy flow transitions from closed to open. When the current value CVxxx is equal to 0, the counter bit (Cxxx) is turned on. The down counter continues to count down to the minimum value (16 bits -32768, 32-bit -2147483648) and stops counting. Performing a reset operation can reset the counter, that is, the counter bit (Cxxx) is set to 0, and the current value of the counter (CVxxx) is reset to the preset value (SV).

be careful:

To assign the instruction initialization parameter 'count target value' to the current counter value (CVxxx), the program needs to perform a reset counter (Cxxx) operation during runtime, regardless of whether the current counter is in a reset state, and the assignment operation needs to be completed before the reset operation.

Counter range: CVxxx=CV0 to CV255; Cxxx=C0 to C255.

Counter Type	Counter Bit Number	Maximum Value	Counter Number
CTD	16 bits	32767	C0-C199
CTD	32-bit	2147483647	C200 – C234
CTD	32-bit (high-speed)	2147483647	C235-C255

Note: Since each counter has a current value, do not set the same counter number to more than one counter (up and down counters with the same number access the same current value)

Input/Output	Data Type	operand	Description
CV	16 bit integer	CV	Counter
SV	16 bit integer	D/CV/TV/AI/AO/K/H/V/Z/FD, bit composite word (X/Y/M/C/T/S), local variable (LW)	preset value

Command table:

NETWORK 000

LD X000

CTD CV0 D0 // The counter starts counting down from the current value CV0, and the set value (SV) is D0

NETWORK 001

LD C0

OUT Y000

图1 CTD