IBM 5110 - Where does the character set live? And other questions.

Robert robert626001 at gmail.com
Wed Jul 12 21:38:08 CDT 2017


On Wed, Jul 12, 2017 at 03:13 AM, Christian Corti
<cc at informatik.uni-stuttgart.de> wrote:
> I hope this is also true in your case. According to your picture, the last
> column of each character is missing. It could be an issue around the shift
> register (e.g. the Display Data register, a latch, cold solder joint,
> ...). I just see that I have the following info on my site (didn't
> remember that detail ;-) ):

Apologies for the delayed reply and for any messy formatting. I seem
to have stopped getting messages from the list, so am building this by
replying to an earlier message and copying and pasting from the cctalk
archive.

It does look like that last column (or possibly two) is missing,
doesn't it? Going back to the info on your excellent site, the shift
register does seem a good candidate. If we assume that it is parallel
in, serial out, then the problem would have to be on the input side,
as all the bits pass through every flip flop (ruling them out) and the
output is flawed absolutely consistently (ruling out an intermittent
fault there). The same reasons militate against a serial input, I
think.

When I next see the board (hopefully this weekend) I'll try to
identify the shift register (which the cross ref on your site will
make a lot easier) and look for dry joints under high power
magnification. I don't have a complete and working logic analyzer and
in any case, the board is not really accessible with a logic probe but
TTL is cheap so, even if I don't see anything, a blind faith
replacement might be worth considering. I note that it's a 10 bit job
and there's nothing in your parts list greater than 8 bits, so I may
be looking at more than one IC.

Side note: It's probably not a good time to try out my shiny new heat
gun that I've never yet used. Maybe save my first go on it for
something more replaceable.

Of course, a parallel input implies a parallel source, so we still
can't rule out an issue with the character ROS. But I do feel like
we're making progress and I now have something to do that's more
focused than just pulling and reinserting boards until the Gold wears
off the pins, so I'm going to call that a step in the right direction.

Also, thanks for the additional scans, which I have already downloaded.

-- 
Robert


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