"COM O 255" for Watchdog on a system with PCD3.Cxxx might cause problems

FAQ #100101

 

This is because the PCD3.Cxxx with base adress 192 does buffer the state of the output 255 (also if there is no real I/O) and the PCD's CPU is reading this buffered state instead of the state of the Watchdog.
Therefore the "COM" instruction (that sets the output acording to its state) always will set the output to the same state. The result of the watchdog not beeing toggled is that the watchdog relais is falling off.

One of the following solutions could be used as workaround for PCD2.M1x0:

-	The Watchdog can be addressed at I/O 511, 767 or 1023 where no PCD3.Cxxx can be placed.
-	Instead of the instruction "COM" a "SET O 255" and later on in the code a "RES O 255" can be used to toggle the Watchdog.