thanks every for reply.in my program, the timer is scan cycle.there are two rungs in program,every rung has a subroutine call the same timer,after the ton instruction,i backup the timer's DN,TT,EN and acc to some tags and before the ton instruction,i restore the DN,TT,EN and acc from the tags i backup them.then i monitor the timer online ,the value "acc" in the second rung increase very slowly.then i exchange the subroutine in rung 1 with rung 0,there are always a slowly increase "acc" in the second rung.