3 Oct
2013
3 Oct
'13
3:49 p.m.
Hi all --
Last night I finally got the Imlac working to the point where I can run
programs more advanced than a light chaser, and I'm using my newfound
abilities to take a closer look at the functionality of the system's core
memory. This is a Dataram "DR-101" system (Docs are on Bitsavers now), 8KW
of 16-bit words and when I got the Imlac all the socketed ICs on the
control PCB were extremely corroded. I outright replaced the corroded ICs
(most of them were too far gone to deal with and the rest were marginal)
cleaned the sockets, and at this point it's fairly functional.
I whipped up a simple program to walk through memory and test every
location (by writing / reading back all possible 16-bit values at every
location). The results are mostly positive, aside from a small addressing
issue (the upper two MSBs of the address from the CPU are apparently being
ignored by the memory's addressing logic so I get 2K repeated over and over
again).
What I did find was that most of the memory I'm currently able to test (the
repeated 2K) tests out fine. However, there are a few addresses in the
210-240 address range that either have dead bits or behave sporadically,
and there's a single bit near the top of memory that's stuck off. The
failures are 100% consistent (even the sporadic failures always show up at
the same addresses) and reproduce identically if the system is "cold" or
has had time to warm up for awhile.
I'm not sure whether to suspect calibration/alignment issues here -- I
would expect failures to be more widespread or have repeating patterns of
some sort if the system was out of alignment. I'm tending to think that
the core plane may actually be damaged in places, which would be very
unfortunate. However, I don't have a lot of experience in diagnosing core
memory failures; so far I've been fortunate -- the systems I have that use
core memory have not had issues with the memory.
Anyone with more experience here have any suggestions?
Thanks as always (and yes, I'm still working on the 8/L current loop stuff
I've just been distracted because apparently I have no attention span and
thanks again for all the help there :)),
- Josh
3 Oct
3 Oct
8:48 p.m.
Date: Thu, 3 Oct 2013 15:49:45 -0700
Subject: Core Memory Failure Modes
From: derschjo at gmail.com
To: cctalk at classiccmp.org
Hi all --
Last night I finally got the Imlac working to the point where I can run
programs more advanced than a light chaser, and I'm using my newfound
abilities to take a closer look at the functionality of the system's core
memory. This is a Dataram "DR-101" system (Docs are on Bitsavers now), 8KW
of 16-bit words and when I got the Imlac all the socketed ICs on the
control PCB were extremely corroded. I outright replaced the corroded ICs
(most of them were too far gone to deal with and the rest were marginal)
cleaned the sockets, and at this point it's fairly functional.
I whipped up a simple program to walk through memory and test every
location (by writing / reading back all possible 16-bit values at every
location). The results are mostly positive, aside from a small addressing
issue (the upper two MSBs of the address from the CPU are apparently being
ignored by the memory's addressing logic so I get 2K repeated over and over
again).
What I did find was that most of the memory I'm currently able to test (the
repeated 2K) tests out fine. However, there are a few addresses in the
210-240 address range that either have dead bits or behave sporadically,
and there's a single bit near the top of memory that's stuck off. The
failures are 100% consistent (even the sporadic failures always show up at
the same addresses) and reproduce identically if the system is "cold" or
has had time to warm up for awhile.
I'm not sure whether to suspect calibration/alignment issues here -- I
would expect failures to be more widespread or have repeating patterns of
some sort if the system was out of alignment. I'm tending to think that
the core plane may actually be damaged in places, which would be very
unfortunate. However, I don't have a lot of experience in diagnosing core
memory failures; so far I've been fortunate -- the systems I have that use
core memory have not had issues with the memory.
Anyone with more experience here have any suggestions?
Thanks as always (and yes, I'm still working on the 8/L current loop stuff
I've just been distracted because apparently I have no attention span and
thanks again for all the help there :)),
- Josh The address decode is not a problem in the arrays.The bit failures may be.Dwight
9:49 p.m.
On 10/04/2013 12:49 AM, Josh Dersch wrote:
Anyone with more experience here have any
suggestions?
Here you can see some pictures from my attempts at repairing some 8L core memories
:
ftp://jdreesen.dyndns.org/ftp/Cores/
I found dead / duff diodes , old solder joints of original repairs that had gone
high-ohmic, and broken cores.
You want to make sure that all diodes have the same forward voltage when measured, and all
wires have the same resistance. ( except for the sense line of course )
good luck should you find broken cores....
Jos
4 Oct
4 Oct
4:38 p.m.
On 10/3/2013 9:49 PM, Jos Dreesen wrote:
On 10/04/2013 12:49 AM, Josh Dersch wrote:
Thanks. That looks like some very intense repair! Lucky that broken
core was on thecorner :). I'm going to have to get a much more powerful
magnifying glass to scope things out on the Imlac's core planes. Here's
a pic of what I have to work with:
https://plus.google.com/u/0/photos/117997069161125071032/albums/58769649993…
and
https://plus.google.com/u/0/photos/117997069161125071032/albums/58769649993…
(where you can see an earlier repair...)
I'm hoping that the cores are OK, but given the way things are failing
it seems pretty likely. I do have a second core set for this machine
(the pictures above are actually of that set); the logic board is in
much, much worse condition but maybe the core plane is OK. I may swap
the two at some point and see ifthe other works any better.
Thanksagain,
Josh
Anyone with more experience here have any
suggestions?
Here you can see some pictures from my attempts at repairing some 8L
core memories :
ftp://jdreesen.dyndns.org/ftp/Cores/
I found dead / duff diodes , old solder joints of original repairs
that had gone high-ohmic, and broken cores.
You want to make sure that all diodes have the same forward voltage
when measured, and all wires have the same resistance. ( except for
the sense line of course )
good luck should you find broken cores....
Jos
6 Oct
6 Oct
12:40 p.m.
On 10/4/13 6:38 PM, Josh Dersch wrote:
On 10/3/2013 9:49 PM, Jos Dreesen wrote:
Looking through the pictures of your machine, it is interesting to note the differences
from
mine (serial #95) to yours (serial #277). Seems like they must have been on an
evolutionary
track in production as they are both labeled PDS-1D. My console is the older triangular
box,
like used with the PDS-1, but mine has LEDs and paddle switches (like yours) vs the
incandescent
lamp modules and toggle switches used on the PDS-1. Yours has the picture frame design
with
the additional selector knob. As I recall, this newer design console was used on the
PDS-4
as well. My keyboard is the same as used on the PDS-1 while yours has a newer style
enclosure
and set of keys. The top (100s) bank of cards on yours are mostly empty and unwired while
mine
contained the core memory and circuitry to drive it. Yours has a swing out door over the
wire
side of the backplane and presumably the large circuit card and possibly the memory are
mounted
on that door. Finally yours has control switches at the top front of the unit which are
either
not present on mine or are located on the back with the connectors. I assume yours
originally
had a desk top mounted to the top similar to mine, but that is only a guess.
--tom
On 10/04/2013 12:49 AM, Josh Dersch wrote:
Thanks. That looks like some very intense repair! Lucky that broken core was on
thecorner :). I'm
going to have to get a much more powerful magnifying glass to scope things out on the
Imlac's core
planes. Here's a pic of what I have to work with:
https://plus.google.com/u/0/photos/117997069161125071032/albums/58769649993…
and
https://plus.google.com/u/0/photos/117997069161125071032/albums/58769649993…
(where you can see an earlier repair...)
I'm hoping that the cores are OK, but given the way things are failing it seems
pretty likely. I do
have a second core set for this machine (the pictures above are actually of that set);
the logic
board is in much, much worse condition but maybe the core plane is OK. I may swap the
two at some
point and see ifthe other works any better.
Thanksagain,
Josh
Anyone with more experience here have any
suggestions?
Here you can see some pictures from my attempts at repairing some 8L core memories
:
ftp://jdreesen.dyndns.org/ftp/Cores/
I found dead / duff diodes , old solder joints of original repairs that had gone
high-ohmic, and
broken cores.
You want to make sure that all diodes have the same forward voltage when measured, and
all wires
have the same resistance. ( except for the sense line of course )
good luck should you find broken cores....
Jos
2:21 p.m.
On 10/6/2013 12:40 PM, Tom Uban wrote:
On 10/4/13 6:38 PM, Josh Dersch wrote:
Yes, see the picture here:
http://yahozna.dyndns.org/scratch/imlac/pds-4.jpg
I believe the monitor that would have gone with my PDS-1D (now sadly
missing) would have looked similar to that one as well.
Every Imlac I've seen pictures of looks different from every other .
There was definitely a gradual evolution over time. I'm guessing that
the 1D I have inherited a bit from the newer 4's design -- the core
memory boards have "PDS-4" stamped on the back of them, even.
Thanks. That looks like some very intense repair!
Lucky that broken
core was on thecorner . I'm
going to have to get a much more powerful magnifying glass to scope
things out on the Imlac's core
planes. Here's a pic of what I have to work with:
https://plus.google.com/u/0/photos/117997069161125071032/albums/58769649993…
and
https://plus.google.com/u/0/photos/117997069161125071032/albums/58769649993…
(where you can see an earlier repair...)
I'm hoping that the cores are OK, but given the way things are
failing it seems pretty likely. I do
have a second core set for this machine (the pictures above are
actually of that set); the logic
board is in much, much worse condition but maybe the core plane is
OK. I may swap the two at some
point and see ifthe other works any better.
Thanksagain,
Josh
Looking through the pictures of your machine, it is interesting to
note the differences from
mine (serial #95) to yours (serial #277). Seems like they must have
been on an evolutionary
track in production as they are both labeled PDS-1D. My console is the
older triangular box,
like used with the PDS-1, but mine has LEDs and paddle switches (like
yours) vs the incandescent
lamp modules and toggle switches used on the PDS-1. Yours has the
picture frame design with
the additional selector knob. As I recall, this newer design console
was used on the PDS-4
as well. 
My keyboard is the same as used on the PDS-1 while
yours has a newer
style enclosure
and set of keys.
Yeah, and it looks more like the one in the PDS-4 picture linked above
(minus the switches on the right hand side.)
The top (100s) bank of cards on yours are mostly empty
and unwired
while mine
contained the core memory and circuitry to drive it.
Additionally, all the unused positions have no slots at all installed --
i seem to recall that all slots are present in your machine. I guess
that must have been a cost-cutting measure. There are unused slots in
mine, but at one time they were filled (it used to have the disk
controller installed in the 300 row, now all that's left is the DMA logic.)
Yours has a swing out door over the wire
side of the backplane and presumably the large circuit card and
possibly the memory are mounted
on that door.
Yes, that's correct. The memory system (two sets of core planes + the
large control PCB) are mounted to the back, along with a separate -18V
supply (and fan).
Finally yours has control switches at the top front of
the unit which
are either
not present on mine or are located on the back with the connectors. I
assume yours originally
had a desk top mounted to the top similar to mine, but that is only a
guess.
Yeah, this was definitely part of some kind of desk. Someday when I
have the space I'd like to recreate it but that'll have to wait...
- Josh
--tom
3876
days inactive
3879
days old
5 comments
4 participants
participants (4)
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derschjo＠gmail.com -
dkelvey＠hotmail.com -
jdr_use＠bluewin.ch -
uban＠ubanproductions.com