HP2100 core memory problems

From: Jay West <jlwest_at_tseinc.com>
Date: Wed Aug 18 20:56:07 1999

Well, once again I come to the well for knowledge.....:) Seeking advice from
those up on core memory

I'm kinda lost on checking out the memory in my HP 2100. I have two 2100's,
each with four 8K boards or 32k per machine. I had lots of parity errors so
I lined up all eight 8K boards on the bench and reconfigured the memory
controller for 8K total rather than 32K (test one board at a time). I put in
one of the boards and ran a small memory test (not a diag tape, but a core
memory test from the CE guide via the front panel).The core test has three
"controlling" locations, one location for the first address to test, one for
the last address, and then how many cycles per memory location. A cycle is
defined as writing all zeros, read, and compare - then writing all ones,
read, and compare all on a single location.

Here's the symptoms. The full test of 8K takes about 2 hours (with a cycle
of 3). On different boards it gets to different locations before the parity
error halt. So far mostly at the middle or end, I don't recall it ever
parity halting towards the beginning. I'll get a parity error halt on say
location 012336 for example. I then manually go to the failing location and
try storing different values in the location and reading them back out (all
via the front panel). Most of the time this gives a parity error like you
would expect. But - many times it doesn't. Then just for kicks I restart the
diagnostic a few locations lower (like 012320 in this example) than the
failing location. It fails right away usually (like location 012322), quite
a few cells before the location which originally failed. This causes me
great confusion, because it's actually now failing on addresses which
previously tested OK just a few minutes ago on the previous test pass.

Every 8K board I have exhibits this problem, but all at different locations.
Supposedly both machines worked fine before they were put in storage 20
years ago, so these weren't picked up out of a junk heap or anything.
Cosmetically they're beautiful inside and out. I've also tried switching the
XY driver boards, the memory controller, etc. etc. but I can't seem to get
anything stable enough to start intelligently swapping parts when every
combination is bad. In case it matters, each 8K board is 17 bits per word (1
parity plus 16 data bits). Also, it isn't a complete failure - I've keyed in
many other programs from the front panel and read in paper tapes (a suite of
I/O tests and such) that all run fine so I know the system is somewhat
coherent. I've checked the power supply for the memory cage and it's
supposed to be 20 volts - my VOM came up with 20.48 ISTR, which is probably
close enough I would think. Can anyone suggest a course of action or
possible culprits in this situation? I suspect I can't see the forest for
the trees anymore :)

Thanks!

Jay West
Received on Wed Aug 18 1999 - 20:56:07 BST