Allen-Bradley Logix5000 Reference Manual page 146

Chapter 3 Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES)
Description:
Rung-condition-in
Count-down Enable Bit (.CD)
Count-down Done Bit (.DN)
Counter Accumulated Value (.ACC)
Arithmetic Status Flags:
Fault Conditions:
146
The CTD instruction is typically used with a CTU instruction that references
the same counter structure.
When enabled and the .CD bit is cleared, the CTD instruction decrements the
counter by one. When enabled and the .CD bit is set, or when disabled, the CTD
instruction retains its .ACC value.
Preset
The accumulated value continues decrementing, even after the .DN bit is set. To
clear the accumulated value, use a RES instruction that references the counter
structure or write 0 to the accumulated value.
Not affected
None
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
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