Part II - PLC Watchdog - design and schematic.
Below is the preliminary PLC WatchDog Schematic we used to build a "missing pulse detector" for our Home Automation PLC using
the Parallax Digital I/O Board. Arduino users can use this "preliminary" schematic as an engineering starting point to prevent equipment and property destruction
by disabling the digital control outputs when the following occurs:
#1. Microcontroller/Gator/Gator+/Arduino device failure.
#2. I2C lockups/hangs due to loss of device +5VDC vdc power or device failure, which hangs/halts the Arduino/CPU.
#3. Loss of device/system +5 VDC power.
#4. Microcontroller uploading which might cause the control digital outputs to turn on.
#5. Microcontroller software bugs or revision changes which could force the digital control outputs on.
#6. Software task timeouts or scan overruns. (Software stops the WatchDOG)
The microcontoller needs two digital outputs to control the PLC Watchdog.
One resets/restarts the WatchDog timer.
The other output pulses or "kicks the dog" to keep the WatchDog enabled via software control.
When the microcontroller fails, device fails or under software control the pulsing stops, all digital control outputs are disabled.
Note: You can use just one digital output but you need to manually reset the timer on all uploading, power blips or brownouts.
You could also prevent brownouts by using a computer UPS, which most modern PLCs already have built-in to their systems.
Note: There are no known problems with the PLC Watchdog ... yet ... further testing in progress.
Note: Watchdog part costs ~ $13 USD.
