Allen-Bradley Logix5000 Reference Manual page 125

Description:
Rung-condition-in
Timer Enable Bit (.EN)
Rung Condition Controlling RES Instruction
Timer Timing Bit (.TT)
Timer Done Bit (.DN)
Timer Accumulated Value (.ACC)
IMPORTANT
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES)
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.
0
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.
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
Timer did not reach .PRE value
Chapter 3
Preset
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