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Compaq SLT/286 4MiB Memory

S

schenkzoola

Member
Joined
Dec 11, 2020
Messages
32
Location
Portland
Hello,

I'm looking for some high resolution pictures of the 4MiB memory modules for the SLT/286 for a reverse engineering project.

I've already got a 1MiB module RE'd, but there are three pins on the edge connector that are not connected. I'd like to see which ones are connected on the larger module, as well as learn what chips were used, etc...

If you have one you can take good pictures of, or better yet loan/sell me, I'd greatly appreciate it!

Thanks!
 
Just so you know I'm serious, here's what I did with the 1MiB module:
 

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That's incredible! I just bought a slt/286 (should be in the mail soon) and would love to get it fully decked out! Unfortunately it didn't come with the battery or power adapter so the pinouts you provided will prove very helpful. Did you end up trying to make a new power supply or replacement battery? And how'd the ram modules turn out? I'd love to make some since they're virtually impossible to find!
 
schenkzoola said:
An update:

I didn't get any replies, so I forged ahead! Here's what I came up with. It's at the fab now!
View attachment 1039792
Click to expand...

Nice and clean design!

Will these be on your github as gerber files or do you plan on selling them fully assembled?
From what I gather the SLT 286 and 386 share the same type of RAM.
I recently gotten a SLT 386s/20 and decided to register on this forum after stumbling upon a thread whilst I was searching if anyone had already made an attempt at reverse engineering the RAM modules.
Thank you so much for the hard work you've put in!
 
DinohScene said:
From what I gather the SLT 286 and 386 share the same type of RAM.
Click to expand...
They actually do not! They look similar, but the slot connector on the 386 modules is more towards the center instead of off to the side like the 286 modules.

It would be awesome if someone started selling complete repro 4MB modules for the 286 and/or 386s/20. I'd definitely buy some!
 
Well I also forged ahead, was able to find someone that owned an original 4meg module who was also a willing assistant. They got me some great high res photos and help with tracing some connections out, and I've reproduced (mostly) the original module. I say mostly because without taking the chips off the board, I had to make some guesses on routing underneath the ICs that I know are just me stitching and not original, but all the lines connect to where they're meant to go so it *should* work!
Unfortunately I can't find original HM658512A even on the grey market, so looking to replace with IS61C5128AS, which should be a suitable modern replacement.

Once I've got it built and tested, I'll release all the Kicad files.

1737185695812.png1737185730423.png
 
Looks like we may have gone from no RAM expansions available to actually having options :) Looking forward to seeing your progress!

Out of curiosity, the lines on pin 22 and 26, where do they go to? Wonder what these resistors are for?
 
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BeniD82 said:
Out of curiosity, the lines on pin 22 and 26, where do they go to? Wonder what these resistors are for?
Click to expand...
The PSRAM ICs are split into two banks of 4x4Mbit for 16Mb each, total being 32Mb for 4MB - the two lines on 22 and 26 go through 33 ohm resistors to the two 74AC138's (U1/U2) which control the /CE for each bank. I guess you could consider them to be A19/A20.
U7 is a 74HC08 which takes the /RFSH and /EN signals as input and then chains all the AND gates together and outputs /OE for the SRAM. I'm told this is as a delay element.
I'm not 100% sure on the 33R's... I'd expect them to be on all address lines to control ringing, so maybe they are present on the SLT's mainboard for the other lines but not those two, so they just added them on board. Just a guess though, I'm no EE!

BeniD82 said:
Looks like we may have gone from no RAM expansions available to actually having options
Click to expand...
While I'm at it and now have full scans of my 1meg module disassembled, I'm going to recreate that also just in case someone wants it!
 
Update time! I finally have a working module!

The first round of reverse engineering was mostly successful, but when the module was assembled and tested it only detected 1MB.
1740193127159.jpeg1740193044505.jpeg

I spent a bunch of time poking and prodding and testing (incl logic analyser) - it appeared that my SLT wasn't even trying to use more than 1MB, so I started to suspect my SLT was at fault.

1740193145206.jpeg

I put the call out to vintage groups, and found a local that let me borrow his SLT to test, and it turned out he had a 4MB module as well! Gave me permission to remove the ICs down to the bare PCB and I got some really high quality scans that I used to correct all the guessed traces underneath the ICs. I also then found that my SLT wasn't at fault, but it turns out that two of what looked to be power pins were swapped between the 1MB and the 4MB modules. The SLT seems to use them to detect the size of the module installed!

A quick bodge later and...
1740193247298.jpeg1740193259881.jpeg

I've got the final corrected PCBs on order now from JLCPCB and have uploaded the Kicad files to Github - https://github.com/silvervest/Compaq-SLT-286-4Meg/
 
Figured I'd give your module a try as well and happy to confirm that it's working like a charm. Didn't have the right capacitors size wise and ran out too but even so works great. Also used an AC08 instead of the HC08 since that was what I had on hand and no issues.
 

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