Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 121
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES) Chapter 3
When the TOF instruction is disabled, the .ACC value is cleared.
A timer runs by subtracting the time of its last scan from the time now:
ACC = ACC + (current_time - last_time_scanned)
After it updates the ACC, the timer sets last_time_scanned = current_time. This
gets the timer ready for the next scan.
Arithmetic Status Flags: Not affected
Fault Conditions:
Rung-condition-in
Timer Enable Bit (.EN)
Timer Done Bit (.DN)
Timer Accumulated Value (.ACC)
Timer Timing Bit (.TT)
0
16650
Timer did not reach .PRE value
Preset
OFF Delay
Make sure to scan the timer at least every 69 minutes while it runs. Otherwise, the ACC
value won’t be correct.
The last_time_scanned value has a range of up to 69 minutes. The timer’s calculation rolls
over if you don’t scan the timer within 69 minutes. The ACC value won’t be correct if this
happens.
While a timer runs, scan it within 69 minutes if you put it in the following:
• A subroutine
• A section of code that is between JMP and LBL instructions
• A sequential function chart (SFC)
• An event or periodic task
• A state routine of a phase
A major fault will occur if Fault type Fault code
.PRE < 0 4 34
.ACC < 0 4 34
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