Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 147
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES) Chapter 3
Execution:
Condition Relay Ladder Action
Prescan The .CD bit is set to prevent invalid decrements during the first program scan.
The rung-condition-out is set to false.
Rung-condition-in is false The .CD bit is cleared.
The rung-condition-out is set to false.
Postscan The rung-condition-out is set to false.
Examine .CD bit.
.CD Bit = 0
.CD bit = 1
Rung-condition-in is true
.ACC Value Rolls Over
Yes
No
Examine .UN Bit.
.UN bit = 0
.UN Bit = 1
.CD bit is set.
.ACC = .ACC - 1
Examine .OV Bit
.OV Bit = 0
Examine .OV Bit
.OV bit = 1
.OV bit is cleared.
.DN bit is cleared.
.UN bit is cleared.
.UN bit is set.
Examine .ACC
.ACC ≥ .PRE
.DN bit is set.
Rung-condition-out is set
to true.
End
.OV Bit = 1
.ACC < .PRE
.DN bit is cleared.
.OV bit = 0
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