The whole carpel tunnel issue is one of correct posture.
When I took typing, back in the dark ages when they were manual, the first thing the teacher did was instruct everyone on the correct posture for using the keys.
You never put your wrists on any part of the machine, you needed to have your hands properly positioned so that you had enough power to drive the keys onto the paper.
When I see people using keyboards, I just cringe, they will definitely have problems in the future do to poor work habits.
Welcome to the community! No problem at all, I have said in other comments on other videos (I think it may have been the CRO) I try and research each product I complete a tear down of, and be knowledgeable on each subject. Of course, this means I can at best be a jack of all trades, master of none. So I for one, appreciaite being corrected where my research didn't yeild the right results, or I out and out mess something up!
Sharing knowledge is what the community is all about, so we are all greatful to have you here!
I still have this one on my "to do" list to recap the power supply and bring it to life again, so the documentation you have linked will prove incredubly helpful for me at least!
The memory expansion card I assume is an after market card, as like you, I found the original was a single "long" card. I would guess that this was made at the same point as the floppy drive replacement?
I think I assume that the clone market originated with Compaq and thier infamous two teams of engineers talking through a team of lawyers! But you are of course right, there were other, less ligitamate clones too!
I've enjoyed your teardowns on Youtube for quite a while now, but today I finally registered at Element 14 so I could post where (as you say every time) you're more likely to see it. Prepare to be overwhelmed with details and corrections...
I found a site with links to many vintage IBM PC (XT/AT) manuals: http://minuszerodegrees.net/manuals.htm#IBM
One of the manuals is the 5155 "Guide to Operations", that has disassembly instructions needed to install the available options, including the "IBM Math Co-processor". (Oddly, the instructions for that say the standard 8088 main processor to be replaced with a new 8088 processor that's "compatible" with the co-processor... no idea what that's about! But regardless...)
The list of available expansion cards shown in the manual - all with pin-out diagrams - include:
memory expansion card;
game control adapter;
and several communications cards for Synchronous, Bi-synchronous, and Asynchronous networks, as well as the "IBM Personal Computer Cluster" adapter.
I've looked through several of the hardware manuals, trying to find anything that explains the stacked memory chips, without any success. However, at the Vintage Computer forum, I found a discussion of a memory expansion board with stacked chips, and one of the authors separated a spare/scrap pair of stacked chips to find that they're different part numbers. Not the same chips as on your card, with one marked "(TI logo) ZA1250NL AP8510", and the other marked "LA/02406H13 SINGAPORE", but certainly a valid way to increase memory capacity by stacking chips.
Pausing the video at various points, and looking at the chips carefully, the paired board with the I/O interface on it has stacked chips marked OKI M37S64A, while the add-on board has single chips that are marked OKI M3764 (without an 'S'), which is the same as on the memory on the motherboard. After a bunch more searching, I eventually found datasheets for both of those at www.minuszerodegrees.net:
The datasheet for the 'S' version shows a picture of two stacked chips so it seems they came from the factory that way. The plain one is a 64Kbit chip, while the stacked package is 128Kbit (2 Stacked 64K chips).
Eventually, after much digging through Google search results, I found references that indicate to me that the card with the stacked chips should be a "Memory Expander with Parallel Adapter". I found an eBay listing described as "IBM 6236194 ISA 128K MEMORY PRINTER ADAPTER WITH 256K MEMORY EXPANSION 61X6611", with pictures showing the same "PN61X6611" and "ECA40227" numbers as your board printed on the PCB. The one on eBay was just a single board (didn't have the pin header), and the chips on it appear to be the non-stacked (no 'S') OKI M3764A, but still, I'm pretty sure that that double interface card is a parallel adapter (*not* serial, as you said) with memory expansion of 128K bytes of ram with 1-bit parity (1 banks of 9 128Kbit stacked chips), and an add-on memory expansion card with another 256K (4 banks of 9 64Kbit chips).
While I'm at it, the board you called a parallel interface card is clearly marked (@16:52) "ASYNC CARD", so I think you'll find that it's actually the Asynchronous communications protocol interface card - so that one is serial, and not parallel: (pin-out available here: https://www.minuszerodegrees.net/oa/OA%20-%20IBM%20Asynchronous%20Communications%20Adapter.pdf)
So it looks like your luggable computer has:
- a "maxed-out" total of 640K ram (256K on motherboard + 128K on interface card + 256K on daughter card);
- a combination memory expansion/parallel (printer) interface card in slot 2 (with a female DB-25 connector to plug the printer cable into);
- and an Asynchronous serial communications protocol interface in slot 3 (with a _male_ DB-25 connector to plug your modem cable into).
And one last note about the ROM contents being "... legally very challenging..." (15:49): The Guide to Operations manual linked above has the complete source listing for the BIOS. This openness is one of the reasons the clone market was able to exist. While some cheats simply copied the code, and were subsequently sued for copyright infringement, anyone could use the listing to figure out the details of the interface and replicate it by writing their own code. With a functionally identical BIOS, and with the same Intel processor & ISA bus architecture, then any software will run on any computer. Et Voila, instant clone market!
"...so that big connector we can just about see from outside, this is what I was alluding to earlier; we now pull that off, we can see it is a proper PS/2 connector for the keyboard..."
It is the original PC 5-pin DIN keyboard connector as was used on the original PC, PC XT, PC AT machines. The 6-pin mini-DIN PS/2 keyboard connector came later, around 1987 with the launch of the Personal System/2 series machines and then commonly used with the smaller laptops.