IBM 5160 oddball fault

tony duell ard at
Thu Apr 23 12:30:24 CDT 2015

> > Yes, the cursor is generated by the 6845 on the CGA card.
> Since the messages start out skewed to the right, I'd consider that
> there is a stuck bit in the data written to whatever 6845 location
> controls that.  If you can probe the bus and the data is okay most
> places, it may be noisy or corrupted to the slots.  Obviously a lot of
> the memory has to work for the bios to pass and run, so that is
> suggesting some problems getting data to the interface slots, though
> most of that is working or the display card would have failed as well.

The cursor actually has nothing to do with where the data is written or 
appears. And I understood the cursor appeard correctly, that is in the 
location you would expect characters to be written, even though they
are not. This suggests the 6845 is being set up correctly but that the
processor is not writing to video memory where it thinks it is.

I guess it's also possible that the 6845 is not being set up correctly
to start the display at the first location of video memory. The problem
then is that the 6845 _is_ being set up correctly to generate valid timing,

> Fault may be on the card data path as well, since so much other I/O is
> working fine.

I believe this CGA card works correctly in another machine.


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