TON not resetting at power up?

[ianbuckley 0]

I have what I hope is not a stupid question. I have a simple bit of logic for a purge air system where the pressure switch must be made for 90 seconds before we allow power to a part of a machine. I am getting information from a field service rep that when power is cycled to the control system (PLC in Remote Run), the time delay does not function. If he cycles the PLC switch to Prog, then Remote, then Run back to Remote, the delay works normally. Since the timer was completed when the system powered down and the enable conditions could be met at power up, do I need to manually reset the timer in the "power up handler" routine to ensure the delay functions when power is cycled? All timers reset as part of the PLC OS at power up, don't they? I have attached a generic version of the code for reference. Mr. PLC question.pdf Edited 12 Jan 2011 by ianbuckley

[b_carlton 61]

The help file shows tthat the 'prescan' resets the TON. Are you sure the PLC is powered off in the condition you are mentioning?

[ianbuckley 0]

I saw the same thing on the help file. That was why I didn't figure I needed to do anything. The field service rep reports that this (no delay) happens when the control system is powered up. Once he cycles the key switch, it works correctly from then on. He is half a world away, so I cannot verify with my own eyes.

[Ken Moore 23]

You can use the first scan bit to ensure the timer is reset. That way you KNOW it's going to be reset. Ken