Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 125
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES) Chapter 3
Description: The RTO instruction accumulates time until it is disabled. When the RTO
instruction is disabled, it retains its .ACC value. You must clear the .ACC value,
typically with a RES instruction referencing the same TIMER structure.
The time base is always 1 ms. For example, for a 2-second timer, enter 2000 for
the .PRE value.
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.
Rung-condition-in
Timer Enable Bit (.EN)
Timer Done Bit (.DN)
Timer Accumulated Value (.ACC)
Timer Timing Bit (.TT)
Preset
0
16651
Rung Condition Controlling RES Instruction
Timer did not reach .PRE value
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
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