Count Down (CTD)

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.
RSL5K_CTD_Count Down
Available Languages
Ladder Diagram
Ladder Diagram_Count Down (CTD)_v1
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 False
CTD Flow Chart (True)
CTD Flow Chart True
Example
Ladder Diagram
FTStudio-CTD-LD-EX-V1
In this example, a conveyor brings parts into a buffer zone.
  • Rung 1: Each time a part enters, limit_switch_3 is enabled for a scan and counter_2 increments by 1. When the part clears, limit_switch_3 is no longer enabled.
  • Rung 2: Each time a part leaves, limit_switch_4 is enabled for a scan and counter_2 decrements by 1.
  • Rung 3: If there are 15 parts in the buffer zone, counter_2.DN becomes enabled and conveyor_A is disabled, stopping the conveyor from bringing in any more parts until the buffer has room for more parts.
  • Rung 0: An operator can manually reset the conveyor system by toggling the restart bit, which resets counter_2’s status and accumulator.
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