The ctd instruction counts downward each time the rung-condition-in transitions from false to true.
Ladder diagram
| Operand | Data Type | Format | Description |
| Counter | COUNTER | tag | Counter structure |
| Preset | DINT | immediate | Value of Counter.PRE. |
| Accum | DINT | immediate | Value of Counter.ACC. |
COUNTER Structure
| Mnemonic | Data Type | Description |
| .CD | BOOL | The countdown enable bit contains rung-condition-in when the instruction was last executed. |
| .DN | BOOL | The done bit when clear indicates the counting operation is complete. |
| .OV | BOOL | The overflow bit when set indicates the counter incremented past the upper limit of 2,147,483,647. |
| .UN | BOOL | The underflow when set indicates the counter decremented past the lower limit of -2,147,483,648. |
| .PRE | DINT | The preset value specifies the value which the accumulated value must reach before the instruction indicates it is done. |
| .ACC | DINT | The 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.
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.
