Count Down (CTD)
The CTD instruction counts downward each time the rung-condition-in transitions from false to true.
The CTD instruction is typically used with a CTU instruction that references the same counter structure.
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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Ladder Diagram
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Operands
IMPORTANT:
Unexpected operation may occur if:
- Output tag operands are overwritten.
- Members of a structure operand are overwritten.
- Except when specified, structure operands are shared by multiple instructions.
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. |
Affects Math Status Flags
No
Major/Minor Faults
None specific to this instruction. See Index through arrays for array-indexing faults.
Execution
Ladder Diagram
Condition/State | Action Taken |
|---|---|
Prescan | The .CD bit is set to true to prevent invalid decrements during the first program scan. |
Rung-condition-in is false | Set Rung-condition-out to Rung-condition-in See CTD Flow Chart (False) |
Rung-condition-in is true | Set Rung-condition-out to Rung-condition-in See CTD Flow Chart (True) |
Postscan | N/A |
CTD Flow Chart (False)
CTD Flow Chart (True)
Example
Ladder Diagram
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.
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