"COM O 255" for Watchdog on a system with PCD3.Cxxx might cause problems
In case there is a PCD3.Cxxx that contains the slot having the base address 240, the watchdog programmed by using the IL instruction "COM" won't work properly.
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.|
PCD3 / Cxxx
PCD2 / M1xx
PCD2 / M48x
Last update: 31.05.2015 08:47
First release: 08.06.2004 15:26