Overview:
Each processor board has a DS1232 Watchdog chip. This chip allows the processor to boot up when the 5 VDC supply is within operating range. This is why we usually check the 5 volt supply before proceeding to other troubleshooting steps.
The watchdog chip will prevent boot up until voltage is between 4.75 and 5.25 volts DC regardless of -12 and +12 VDC supply.
Other peripheral boards, like I/O, Output, A/D and D/A board don't monitor voltage.
The processor monitors the 5 vdc supply to prevent "bad decisions" due to under voltage by "holding" the processor until the voltage is ready for operation. Of course, this only takes a second when the machine is powered up.
Other boards start working when the processor tells them what to do.
While the processor is running the watchdog also monitors the action and if the processor gets "stuck" and stops executing instructions, the watchdog will catch the interruption and reset the processor. In this case there could be a pin out of position on RAM or ROM, or just a faulty chip on the board.
Troubleshooting:
Deeper investigation reveals that you can measure the signal on the watchdog circuit on the processor board. This can be used to determine if the problem is just a display board issue or a processor problem.
The IC number of this chip will vary with each processor board, but the testing procedure is the same. Locate the chip and measure pin 6 to DC ground. If this voltage is stable, at 5 VDC, then the processor is running. If the voltage is fluctuating from 0 to 5V, then the processor is NOT running.
Checking voltage on Watchdog chip using pin 6.
Note this IC chip number and placement may be different for your board.
