S_ODT Timer reset

From the Info the timer only ever resets if the input R input is set, so it retains the value from the previous time that the S input was high and should therefore trigger on the next input that takes the timer through its time. ie for a 4 sec time one input of 3 sec and the next of 2sec.

If I am examining a heartbeat coming in and checking for the heartbeat on and off (broken wire/comms) how do I ensure that the timer is reset to zero before the next on and off beat?

I am guessing that you are using a Siemens S7.

Usually a heartbeat will not time out if everything is communicating fine. The master doing the heartbeat will reset the timer before it expires. This is done by usually writing the present value of the timer so it will not time out.

https://support.industry.siemens.com/tf/ww/en/posts/timer-s-odt/102762/?page=0&pageSize=10

Hope this helps you out.
Regards,

Also look into IEC Timers which dont latch such as TON

The S_ODT timer is not retained.

If it is a 10 second timer every positive edge it will start counting down from 10 again. Not where it left off.

@Joltran Thank you that's the info I was looking for, means my heartbeat fault is genuine but probably due to an ongoing network or wiring issue (especially as it has mysteriously disappeared recently after the network monkeys have been fiddling)