[ischmitz]

You have a good working DME. Use your oscilloscope and measure the ALE signal. You need to be able to see the 1 MHz clearly. And then do the same on the other DME. There shouldn’t be any triangle signal whatsoever. The ALE signal is required for the 8051 to access its external address space.

The watchdog circuit monitors the ICV output of the 8051. If the program is properly running there is a square wave and that satisfies (feeds) the watchdog. If for some reason this square wave doesn’t get produced the watchdog triggers and brings the reset pin to logic high and then after a certain period brings the reset pin to logic low. This resets the 8051 and it starts to execute code from the reset vector. The reset vector is located at 0x0000.

In your case the watchdog keeps resetting the 8051 because (1) there is no activity on the ICV output or (2) because the watchdog circuit itself is damaged and doesn’t recognize the ICV signal.

If you trigger the oscilloscope on the high-to-low transition of the reset line while measuring the ICV output you should be able to tell what case you’re dealing with.