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

FAQ #100101

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

Views: 3723

The requested software / document is no longer marketed by Saia-Burgess Controls AG and without technical support. It is an older software version which can be operated only on certain now no longer commercially available products.