HP 2100A Restoration

Jay West jwest at classiccmp.org
Tue Aug 2 18:33:18 CDT 2016

Glen wrote...
Isn't there a looping memory test capability built in through the front panel? Maybe you have to flip a switch or something to enable that. I'd have to look at the manual to remind myself.
My foggy memory is telling me the same thing as Glen mentions above. The 21MX (m/e/f) definitely has a built in microcoded memory test, but I don't recall 100% on the 2100.

Hayden wrote....
Well I'm hoping for the memory section that I can find at least one of the
4 core boards that works right away. I've got an 8k driver board I can swap it so I could run the system on only a single core board if needed.

That's the problem, what I was referring to earlier. You can't really test just a 4K board. Its been a long while since I had to pop the cover on a 2100 thus my recent brain memory is all 21MX stuff. But as I recall a minimal 2100 memory section is memory controller, ID, XYD, and core. You will be unlikely to tell which of the 4 is causing issues just with front panel tests. A typical program doing read/write/compare can fail with no indication which of the 4 boards is really the problem. Swapping is problematic if there are several bad boards. You can also run into a difficult to test situation if a 16K ID board is bad "at the top", meaning an 8K board tests ok, but the same board tests bad if it's at "the next 8K". Or if you add another 8K, determining if it's the 8K or the upper part of the 16K ID that's faulting.

Troubleshooting the memory of a 2100 via boardswapping with no known good boards is ... irksome. Understatement of the year.

One thing to keep aware of (this is sketchy, as it's very foggy memory)... I think there is a switch on the panel (under the cover I think) that lets you clear or set all memory INCLUDING the parity bit (memory is 17 bits on these). If you don't do this between memory tests - and you have some board that is not setting memory parity correctly for each word, you are really gonna be scratching your head. Once you get bad parity bit status, you have to wipe before retesting.

One other thing I remember from working on the 2100's.... there was some setting - probably power supply voltages, or something with the memory section - that was temperature sensitive. You had to adjust some trimpot or the like based on the room temperature (+/- a window of course).


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