Author Topic: Make the Serial Watchdog Auxillary (AUX) Relay act like a Normally Closed Relay  (Read 2608 times)


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A lot of customers use the Internal Serial PC Watchdog for timing and alerting purposes rather than rebooting the PC. Typically they use the AUX relay to trigger an external device when the Serial PC Watchdog times out as a result of an application program or OS malfunction.

The relay on the board is a normally open (SPST-NO or 1 form A) type relay, but the customer requirement is for a normally closed type that will open when the board times out.

There is a simple solution to this requirement:

    * Use the BPI_Config program IO Options screen (or your application program) to set the Aux relay to Invert at Power Up.
    * Also set the Aux Relay at Reset Option for: Pulse.
    * When PC power is applied the Serial PC Watchdog will turn on the relay within 200mS so it "acts" like a normally closed type relay.
    * If your application dies and stops "tickling" the watchdog, it will timeout and pulse the AUX relay to on. Of course it is already on at that point.
    * At the end of the pulse time (3 second default) it will turn off the Aux relay and trigger the external device.

If you write your own application program for the PC you could use the DLL function call to turn the AUX relay back on if needed.
« Last Edit: September 28, 2010, 13:04:40 by Admin »