Compaq Portable II, Recreating the 512/1536 Kbyte System Memory Board (104176-001)

[btb]

The Portable II (and maybe other systems?) were able to expand their RAM without using up an ISA slot by attaching this board below the motherboard. I am guessing that not many of them were made because my web searches have turned up basically nothing. With there being only 4 ISA slots on the motherboard (only 2 of those 16-bit) I think it would be nice to reproduce one.

The Portable II service manual has this lovely rendering of the board:


So aside from the (1x256K) DRAM chips there are just four other ICs, two resistor networks, and other minor components. The motherboard itself has RAM Bank 0 (eighteen 1x256K chips) and Bank 1 (four 4x64K chips and two 1x64K chips).

The connector on the bottom of the motherboard is an AMP 102766-4. Although it looks like a 60-position housing, it only has 52 connectors, the outer 4 holes on each side being empty.

So the mating connector would be the AMP 5-102871-1:

But probably any reasonably tall header would do.

Here is the pinout as I was able to trace it so far, maybe it looks familiar to somebody?


The data lines are just direct to the ISA bus, so I think a couple of those other chips are transceivers, and the address lines are the same multiplexed lines used by the RAM banks on the motherboard (the address line ordering might be jumbled a bit, but I think it mostly doesn't matter).

What other lines should I be looking for? Maybe there's a similar-working expansion board out there that I could copy.

Is there a schematic out there for the Portable II? I know there's a SAM's for Compaq Portable, but that doesn't cover the II does it?

 

[btb]

Made some surprisingly nice progress reverse engineering the motherboard. I found the three RAS lines (one for each bank) and CAS lines (one for high byte, one for low byte), parity bits in and out, and most of the select logic for the transceivers on the expansion board.

I even found what looks like a tenth address line on the compaq connector, so maybe I can use higher-density chips (or SIMMs) in my project...

 

[VERAULT]

Is this the board that brings my Compaq Portable II up to 2.5mb?.... Or is it 3.5MB?

Just an added thought. The power supply unit (with card edge connector) on the model II is super wonky. Lots of folks, myself included have issues with those. Mine will boot when it wants too.. That would be a nice replacement. Another member send me some pinouts from his when mine was shot a few years back.. But I was able to get mine working.. enough that I abandoned a replacement psu.

 

[btb]

The board as described in the service manual adds 1.5MB, so the total would actually be 2.125 I guess? 2.5 if you include the 384k for the bios area. There’s some small chance that the connector would allow for more than this though.

I’ve wondered about the supply too. I have two systems with at least one good supply, I haven’t checked the other one yet. Is it the little daughterboard that goes bad I wonder?

 

[VERAULT]

Its been a while since I booted mine but that number seems correct. I swear I thought I had the RAM on a 16bit ISA card from compaq.


I really cant say on the psu, its just not straightforward in design.

 

[btb]

Yeah, you probably have the "512/2048 KByte Memory Expansion Board" (part 105033-001) which is an ISA card that was also an option from Compaq. The board I'm working on was the supposed alternative to that that didn't take up any slots - but I've never seen one or even a photo of one, so maybe it was never produced.

 

[HoJoPo]

My Portable II has this memory card, here's some photos of it. 1664K total system memory currently, I need to find my 150ns 41256 chips to put another 512k on it.

 

[HoJoPo]

Oddly, the part number on this crosses to an ISA hard drive controller for the Deskpro 286 (000330).

U7-U54 on the diagram above are all 41256 chips, soldered in place. Banks 3 and 4 are socketed, U1-U6, U9-U14, U17-U22, U25-U30, U33-U38 and U41-U46 in each row. The other chips are as follows:

U55 - SN74ALS245AN TI chip, 8731 date code, made in Malaysia
U56 - SN74ALS245AN TI chip, 8731 date code, made in Malaysia
U57 - SN74S734N, MMI logo?, 730823 date code?
U58 - 74F32, F logo (Fairchild?), 8734 date code, made in Singapore

 

[btb]

Wow, thanks for sharing! This will be very helpful!

 

[HoJoPo]

I can confirm the connector on the memory board is that tall AMP connector, it has the AMP logo. Soldered memory are all NEC Japan D41256C-15 with date code 8637FU039, so all 18 are from the same batch. Bank 3 is populated with a mix of NEC Japan/Singapore and a single UK chip, dated mostly in 8710/8706, the UK chip is 8649, and one of the Japan chips is 8523.

 

[HoJoPo]

The individual bypass caps for each 41256 are labelled "475 E5Z" (E five Z). The bigger staggered caps down the middle are "106 +15 K", the K has a line above and below, might be a manufacturer (C27, C29, C32 - C39).

C1, C10, C20, C40 and C53 at the left end are "739 RHL". The resistor packs Z2 and Z3 above the 74ALS245 chips read "10x-1-822 date code 8739" I'll see if I can figure out the color codes on the three individual resistors.

 

[HoJoPo]

R3 is Brown, Green, Brown, Gold (150 ohm, 5%)
R9 and R10 are Orange, White, Red, Gold (3.9k ohm, 5%) R9 and R10 are installed with the color codes opposite each other.

 

[btb]

If you could get some more highly-lit photos of the front and back, that would be awesome, especially the right third of it (the connector side) - backlit might be very useful too (like in front of an all-white computer screen) - if you happen to have a flatbed scanner, a scan of the back side would be very nice.

RE: the part number, I wasn't really sure what to call it. 000330 is the "assembly number", but 104176-001 is the "part number" according to the service manual, but maybe that's more what I would think of as a catalog number.

 

[btb]

The 74S734 is an unusual chip, not really a normal part of the 7400 series. Found it in the MMI datebook (thank you Bitsavers!) and it's similar to a 74S244 except it's supposed to have better characteristics for driving dynamic ram chips.

 

[HoJoPo]

I was able to get a scan of the backside, of course it won't focus on the front. I'll have to dig out my tripod tomorrow to try and get some better photos of the front.

 

[HoJoPo]

Here's some macro shots on my nightstand.

 

[btb]

These are so great, thank you!

 

[HoJoPo]

I just realized that 1664k = 1024k+640k. So fully populated the system memory will be 2176k, with 1536k on the expansion board.

This board uses 18x41256 per bank for 16 bit banks with parity for each 8 bits.

 

[HoJoPo]

I have had an A2 Heaven 8M Piggyback memory expansion for the RamWorks III on my Apple //e for a few years, but kept putting off installing it because to get the full use of the ramdisk capabilities, I'd have to pull all the 41256 chips from the RamWorks III itself (and I already had an AE 2Meg Expander Plus installed, so 3 meg was plenty for a //e).

Upgrading my Portable II gave me a good reason to do so, and 24 of the memory chips are NEC D41256C-15 chips just like were already on my Portable II memory expansion. So I have taken 18 of those and upgraded it to the full 1536k now.

Do you need any more pictures? If not, I'll install it back on my Portable II. The stand offs aren't in bad shape, but almost 40 year old plastic is getting brittle, I don't want to remove/re-install it too many times if I don't have to.

Edit: By the way, Compaq made some quality stuff back then, the 16 pin DIP sockets are all AMP branded as well.

 

[HoJoPo]

Looking at it with a flashlight, it's a 3 layer board with a ground plane sandwiched in the middle. So the top and bottom should show all the circuit traces, there's just a big copper layer in the middle with circles cut out for the components / sockets and vias.