Reseting IEC Timers on Older Siemens PLC's (S7-300/S7-400)

More of a personal note than a blog, but hopefully useful for some weary internet traveler in the future. Problem Resetting IEC Timers for older Siemens PLC's is a bit of a pain. They seem to behave a little differently inside or outside of case statemenents. In newer CPU we can use RESET_TIMER(TIMER) but this is not avaliable eg: RESET_TIMER(#IEC_Timer_0_Instance); So we are forced to do a little more work Solution Setup the timer as normal #IEC_Timer_0_Instance(IN:=TRUE,PT:=T#5s); Then use the following fairly verbose setup to reset it. // This is required to reset the timer on Older CPUs. // In newer CPU we can use RESET_TIMER(TIMER) but this is not avaliable here. // RESET_TIMER(#IEC_Timer_0_Instance); #IEC_Timer_0_Instance.IN := FALSE; #IEC_Timer_0_Instance.PT := T#0s; #IEC_Timer_0_Instance.Q := FALSE; #IEC_Timer_0_Instance.ET := T#0s; #IEC_Timer_0_Instance.STATE := 0;

May 12, 2025 - 14:15

Reseting IEC Timers on Older Siemens PLC's (S7-300/S7-400)

More of a personal note than a blog, but hopefully useful for some weary internet traveler in the future.

Problem

Resetting IEC Timers for older Siemens PLC's is a bit of a pain.
They seem to behave a little differently inside or outside of case statemenents.

In newer CPU we can use RESET_TIMER(TIMER) but this is not avaliable
eg:

RESET_TIMER(#IEC_Timer_0_Instance);

So we are forced to do a little more work

Solution

Setup the timer as normal

#IEC_Timer_0_Instance(IN:=TRUE,PT:=T#5s);

Then use the following fairly verbose setup to reset it.

// This is required to reset the timer on Older CPUs.
// In newer CPU we can use RESET_TIMER(TIMER) but this is not avaliable here.
// RESET_TIMER(#IEC_Timer_0_Instance);

#IEC_Timer_0_Instance.IN := FALSE;
#IEC_Timer_0_Instance.PT := T#0s;
#IEC_Timer_0_Instance.Q := FALSE;
#IEC_Timer_0_Instance.ET := T#0s;
#IEC_Timer_0_Instance.STATE := 0;