Rockwell Automation Publication 1756-RM003N-EN-P - October 2011 135
Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES) Chapter 3
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: None
Execution:
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
Condition Function Block Action Structured Text Action
Prescan No action taken. No action taken.
Instruction first scan EN, TT and DN are cleared.
ACC value is set to PRE.
EN, TT and DN are cleared.
ACC value is set to PRE.
Instruction first run EN, TT and DN are cleared.
ACC value is set to PRE.
EN, TT and DN are cleared.
ACC value is set to PRE.
EnableIn is cleared EnableOut is cleared, the instruction does nothing, and the
outputs are not updated.
N/A
EnableIn is set When EnableIn transitions from cleared to set, the
instruction initializes as described for instruction first
scan.
The instruction executes.
EnableOut is set.
EnableIn is always set.
The instruction executes.
Reset When the Reset input parameter is set, the instruction
clears EN, TT and DN and sets
ACC = PRE. Note that this is different than using a RES
instruction on a TOF instruction.
When the Reset input parameter is set, the instruction
clears EN, TT and DN and sets
ACC = PRE. Note that this is different than using a RES
instruction on a TOF instruction.
Postscan No action taken. No action taken.
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