What is the easiest way to monitor whether a PC has crashed? Can I have the PC transmit a pulse every few seconds, and use a PLC to monitor for that pulse (and shut down the process if the pulse is not detected)?
Thats one way. A very easy way is to have the PLC run a timer, with the PC constantly writing zero to the accumulator of the timer. If it times out, the PLC can assume loss of communications with the PC due to crash, comm
disconnection, or someone has exited the program. Still another way is for the PLC to set a bit, the PC resets it (or vice versa). If it remains set for too long, the PLC assumes loss of communications with PC.
The pulse method you mentioned is used in industry, it is called a 'heartbeat' by some. Basically, one way to do it is to have a counter in the plc which is counting up. The PC periodically resets the counter to zero. If the counter reaches a certain value the PLC can assume the PC, or the application on the PC, is no longer alive and can alter the process.
There are also Watchdog cards for PC's which, in the event of a lock up or failure, will restart the PC.
The "easiest" or "best" will depend upon what you want to have happen as a result of the monitoring and how reliable it has to be.
I have seen watchdog modules that plug into Opto-22 SSR boards. If the computer does not send a regular pulse on that output to reset the timer, the watch dog relay closes (or opens) a contact which you can use to do whatever it is you need to do.
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Michael Griffin
London, Ont. Canada
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