21 Dec
2005
21 Dec
'05
5:11 a.m.
12/20/2005 at 10:45 PM Jim Leonard wrote:
RAM checks out okay after 65536 pattern writes and
reads. So whatever is
making the single-bit errors I don't know -- it only happens in 320x200
graphics mode, and is fine elsewhere. This is an original IBM CGA, not a
clone.
That's what I was working from. Have you tried doing a read of the display
every so often (without writing) and comparing to previous reads to see if
bits are getting picked/dropped? That would settle the question of the
RAM. I'm not sure if a read/write will find all problems.
The CGA wasn't a bad design, considering what was available, although I
wonder about using 74153's in U9/U10 rather than the LS or S parts.
Now the MDA was a mess--traces going nowhere, logic that went nowhere.
I've wondered if it was some incomplete design rushed to production.
Cheers,
Chuck