22 Sep
2014
22 Sep
'14
11:16 p.m.
On 2014-Sep-22, at 9:49 PM, Josh Dersch wrote:
On 9/6/2014 1:27 PM, Josh Dersch wrote:
? right, I call it Shiny Object Syndrome - look! another three-letter acronym.
On 9/6/2014 1:06 AM, Brent Hilpert wrote:
A brief update (and request for further information): I finally had time tonight
to sit down and dig into this (I've been busy and easily distracted by new shiny
objects as of late). On 2014-Sep-06, at 12:53 AM, Brent Hilpert
wrote:
Thanks for all the info -- the core memory overview you put together looks to be
very helpful, and when I have some time (hopefully this evening) I'll study it a bit
more in-depth and see what diagnosis I can apply to the 11.
Thanks,
Josh
On 2014-Sep-05, at 9:11 PM, Josh Dersch wrote:
> Basic testing (via the simple address test listed here:
http://www.psych.usyd.edu.au/pdp-11/hints.html) reveals that most of the 8KW of core
(H215) and controller is working pretty well, except for a 128 byte range between
012000(8) and 012200(8). Within this range, all words read back as all zeros regardless
of what is written.
For 8K the most probable matrix size is 128 * 64 (binary pair closest to the square
root of the size).
Should have mentioned.. it could be a 64*64 matrix with
anti-coincidence addressing to further double it.
Thanks to Brent's writeup and DEC's
documentation (both of which are well written and relatively easy to understand) I'm
pretty sure I've traced down the fault to a diode on the core plane board (H214),
contained within a DIP package designated as a DEC 2501.
Of course, I don't have any of these going spare (and I'm not sure if there are
equivalents -- I have yet to find a datasheet). Anyone have any information about these,
have any (good) spares, or know where I might find some replacements? I suppose I could
just hack in a single diode (assuming I can determine the specs) but that seems kind of
ugly...
Great, thank goodness it looks like a diode problem and not a matrix-wire.
I looked into the diode packs back during the earlier discussion, the best match I found
(based on pinout) for the address-wire current-steering diode pairs (14-pin DIP) was:
TID125, TID126
http://www.datasheetarchive.com/dl/Scans-026/ScansUX10116.pdf
The DEC schematic indicates the pair-pins but doesn't seem to indicate the two
common-anode/cathode pins on the packages, the TID125/126 I think were the only type(s)
from the above page that made sense.
( DEC schematic: page 134 of 1105_RevAH_Engineering_Drawings_Jul76 from bitsavers, sheet 2
of H214-0-1 ).
There's a small chance I might have a replacement or equiv on a scrap board.
Worst case - should be able to use a couple of discrete diodes to sub for the bad ones if
you can ensure the bad ones are out-of-circuit.