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Count Down (CTD)

The CTD instruction counts downward each time the rung-condition-in transitions from false to true.

Ladder Diagram
OperandData TypeFormatDescription
CounterCOUNTERtagCounter structure
PresetDINTimmediateValue of Counter.PRE.
AccumDINTimmediateValue of Counter.ACC.
COUNTER Structure
MnemonicData TypeDescription
.CDBOOLThe countdown enable bit contains rung-condition-in when the instruction was last executed.
.DNBOOLThe done bit when clear indicates the counting operation is complete.
.OVBOOLThe overflow bit when set indicates the counter incremented past the upper limit of 2,147,483,647.
.UNBOOLThe underflow when set indicates the counter decremented past the lower limit of -2,147,483,648.
.PREDINTThe preset value specifies the value which the accumulated value must reach before the instruction indicates it is done.
.ACCDINTThe accumulated value specifies the number of transitions the instruction has counted.


When rung-condition-in is set to true and .CD is false, .ACC will be decremented by one. When rung-condition-in is false, .CD will be cleared to false.

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A conveyor brings parts into a buffer zone. Each time a part enters, limit_switch_3 is enabled and counter_2 increments by 1. Each time a part leaves, limit_switch_4 is enabled and counter_2 decrements by 1. If there are 100 parts in the buffer zone (counter_2.dn is true), conveyor_A turns on and stops the conveyor from bringing in any more parts until the buffer has room for more parts.