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HP 41C Video interface test. Howto?
07-20-2020, 12:31 PM
Post: #27
RE: HP 41C Video interface test. Howto?
(07-17-2020 08:53 PM)Dave Frederickson Wrote:  ...
BTW, what's the difference between the ASCII code for a space and a triangle?


the triangle is non-standard and could be anything (e.g. on the 82143 printer its 8, while on the HP-86B is 7). Its not mentioned in the manual of the 82163 as available.

So we must assume that if the video controller is working, then the code for the triangle must be one that the 82163 considers unprintable (i.e. anything from 0 to 31 and 127).

My guess is that the video RAM is all FFs and the video controller prints a triangle when it sees FF.

Now the video RAM is big enough to store just the ASCII map (31 lines with 32 chars per line) of the screen, leaving only 8 bytes free. So I do not see the CPU requiring access to these 8 bytes in order to work (it has its own internal RAM).

Nevertheless, my chief suspect is still the RAM, but to see what is actually going on, I'd use a logic analyzer to look at the signals being exchanged between the CPU and video RAM, to see if a RAM error is causing the CPU to stop.

Then I'd look to see if the HP-IL IC is working. There are some simple tests you can do, described in my HP-3421A repair video (

Complex stuff and perhaps not really necessary as your only hope of getting this to work is that the CPU is OK. So, frankly, if I were in your shoes, I'd have already replaced the ram.


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RE: HP 41C Video interface test. Howto? - vassilisprevelakis - 07-20-2020 12:31 PM

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