Logix5000 Controllers General Instructions - SLAC Confluence

Timer and Counter Instructions (TON, TOF, RTO, TONR, TOFR, RTOR, CTU, CTD, CTUD, RES) 101When the TOF instruction is disabled, the .ACC value is cleared.rung condition intimer enable bit (.EN)timer timing bit (.TT)timer done bit (.DN)presetOFF delaytimer accumulated value (.ACC)0 16650timer did not reach .PRE valueHow a timer runsA 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.IMPORTANTMake sure to scan the timer at least every 69 minutes while it runs. Otherwise, the ACC valuewon’t be correct.The last_time_scanned value has a range of up to 69 minutes. The timer’s calculationrolls over if you don’t scan the timer within 69 minutes. The ACC value won’t be correct if thishappens.While a timer runs, scan it within 69 minutes if you put it in a:• subroutine• section of code that is between JMP and LBL instructions• sequential function chart (SFC)• event or periodic task• state routine of a phaseArithmetic Status Flags:not affectedPublication 1756-RM003I-EN-P - January 2007