Compressor controls circuit "Watchdog timer"

[powersoff]

What could the purpose of a watchdog timer that is restarted from the output of a PLC in a compressor controls circuit be?
I am searching for clues to help me understand a control panel with incomplete prints and undocumented plc code.
Thanks in advance for any replies.

 

[blutfort]

Sorry if I'm telling you stuff you already know...
A watchdog timer is a timer setup to reset a device should the "Timer" think the device has "Hung". If the watchdog timer expires it probably cycles power to the PLC, or grounds a Reset Pin.
The PLC needs to tell the watchdog timer that it is still awake and not "Hung/Crashed/Loop Locked/Etc...".
I'm used to dealing with the watchdog timers in milliseconds, so the example I'm about to give may not have relevant time scales...
EX:
The watchdog timer has an interval of 10 seconds. If it expires, the watchdog timer resets the PLC, Lights a lamp, phones home, etc...
The PLC is programmed to reset the watchdog timer every 8 seconds. So in theory, the watchdog timer never has a chance to expire.
It might be used if the PLC in question is looking for data from a communications device and there is no way to detect a timeout. It might have been installed if the code in the PLC is really slopply and works 90% of the time but just seems to crash every now and then.

Why it is on your particular application I can't tell you without a lot more info.
Good Luck with your project!

 

[imok2]

The sequence of events for the compressor have to be done in certain time otherwise you have a default and the watchdog timer has to reset the control PLC to the beginning, but as long as the compressor sensors stay normal then the the watchdog gets reset in whatever tome it's programed for. Here's some good reading

http://www.netrino.com/Embedded-Systems/How-To/Watchdog-Timer