Allen-Bradley Logix5000 Reference Manual page 118

Chapter 3 Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES)
Condition
Prescan
Rung-condition-in is false
Rung-condition-in is true.
Examine .DN
Bit
.DN Bit = 0
Examine .EN Bit
.EN Bit = 1
.TT bit is set.
.ACC = .ACC + (current_time - last_time)
last_time = current_time
.ACC Value Rolls Over
Yes
.ACC = 2,147,483,647
Postscan
118
Execution:
Relay Ladder Action
The .EN, .TT, and .DN bits are cleared.
The .ACC value is cleared.
The rung-condition-out is set to false.
The .EN, .TT, and .DN bits are cleared.
The .ACC value is cleared.
The rung-condition-out is set to false.
.DN Bit = 1
.EN Bit = 0
No
The rung-condition-out is set to false.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
.EN bit is set.
TT bit is set
last_time = current_time
Examine .ACC
Rung-condition-out is
set to true.
End
.ACC ≥ .PRE
.ACC < .PRE
.DN is set
.TT bit is cleared