Allen-Bradley Logix5000 Reference Manual page 147

Condition
Prescan
Rung-condition-in is false
Rung-condition-in is true
.CD Bit = 0
Examine .CD bit.
.CD bit = 1
.UN bit = 0
Examine .UN Bit.
.UN Bit = 1
Postscan
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES)
Execution:
Relay Ladder Action
The .CD bit is set to prevent invalid decrements during the first program scan.
The rung-condition-out is set to false.
The .CD bit is cleared.
The rung-condition-out is set to false.
.CD bit is set.
.ACC = .ACC - 1
.OV bit is cleared.
.DN bit is cleared.
.UN bit is cleared.
Examine .OV Bit
.OV Bit = 1
The rung-condition-out is set to false.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
.ACC Value Rolls Over
No
.OV bit = 1
.OV Bit = 0
.ACC ≥ .PRE
Examine .ACC
.ACC < .PRE
.DN bit is cleared.
.DN bit is set.
Rung-condition-out is set
to true.
End
Yes
Examine .OV Bit
.OV bit = 0
.UN bit is set.
Chapter 3
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