Thread by @TubeTimeUS

picked up an interesting SCSI card. it's not PCI, nor is it ISA. it is MCA, and it's gonna go into my PS/2 model 50Z!

the 50Z has no power supply plugs inside since power comes over only on the various edge connectors. ESDI hard drives and PS/2 floppy drives have extra pins just for this reason. my SCSI2SD does not, so i had to solder a jumper wire to the SCSI card.

here's the PS/2 drive sled with a piece of acrylic to match up the mounting holes for the SCSI2SD.

it just screws in right on top!

here it is inside the Model 50Z. this machine can be assembled and disassembled completely without any tools.

so what operating system should i install? this computer has a 286, so that does limit the list somewhat.

the twitter has, err, tweeted and i am to install OS/2 on this beast. however, the Model 50Z has 1MB RAM and OS/2 1.3 requires 2MB. i think i'll try anyway. 🤷

nope. 😐

time to completely field strip my PS/2 Model 50Z. no tools required!

oh look a memory SIMM

well there's the problem

*sigh* do i want to try out random 72 pin SIMMs to see if they work?

got all the pins mapped out. and i found something really weird.

this SIMM has 12 chips on it. each chip has 4 data lines and is therefore a 256Kx4 memory. only 3 data lines of each chip are actually used, so this 1MB SIMM actually contains 1.5MB of memory!?

(this SIMM has parity, so it uses 36 bits of the 48 available from the RAM chips)

another way to look at it: they could have used 9 chips (9*4=36) but instead they have 3 extra chips, so there's 384KB of extra RAM on board. 🤷

aaand the DRAM chip pinout is non-JEDEC. 🤦

near as i can tell, the only difference between the 1MB and the 2MB SIMM is that the 2MB SIMM adds the RAS1 and RAS3 lines. can't be an extra address line because both A9 and A10 are n/c on the planar (motherboard).

bought a few 72-pin SIMMs today. i figure i can modify one to work. some of these are pretty strange...

this weird little chip, on a memory SIMM, has patent numbers on it! it emulates parity bits.

i guess you could save some money at the expense of reliability?

so im going to try and modify this 8MB SIMM to appear as a 2MB SIMM. like the 2MB version, it is also dual rank but it has one extra address line.

first, i will tie A9 low (the extra address line) using a 1K resistor. the computer shouldn't be driving this pin but just in case im using a 1K resistor.

second, ill strap the mpd pins (memory presence detect) to indicate a 2MB 85ns SIMM. there are handy jumpers on the board that allow this.

aaaand nope. 201 is memory error. location 000000. looks like bit 33, which is a parity bit. guess the magic parity chip doesn't work. 🤦‍♂️

gonna try a different SIMM. this one is slower, only 80ns. but first I have to change the ID jumpers on the far right (0 ohm resistors)

that one didn't work. maybe this one?

no luck either, but then I noticed that the original SIMM won't work anymore, and the socket isn't doing so well. 🤔

new socket! let's see if it works now.

that's good. now back to trying out those other SIMMs. 🤪

time to try my original modded 8MB SIMM...

oh wow! the modified SIMM works! at least the BIOS can test it. all I've done is strap an address line low (A9) and changed the 4 ID pins.

now i have to run the reference disk again.

now what was i doing? oh yeah, installing OS/2. 😂

for those who want to replicate my experiment: what i needed was a *dual rank* SIMM with parity. i took an 8MB SIMM and strapped A9 to gnd with a resistor, making it 2MB. a 4MB or 16MB SIMM will not work because those are single rank (i.e. 2 RAS lines, not 4)

uh oh, the installer doesn't support the SCSI card! looks like i need a driver disk from Future Domain: FDU-OS/2. 😐

going to try and revive the old ESDI disk. i replaced a capacitor but the second one has leaked.

not a good sign.

🤮

much better. now to find a replacement capacitor...

all done! i wonder if it works now.

it works! time for OS/2. 😁

IBM blue.

it's gonna be lots of disk swapping.

installed! now i just have to figure out how to reboot 😂

here we go!

and we have graphics!

DOS? 🤔

nice, it looks like Windows 3.1. but this is OS/2 1.3!

woah, they have italic fonts!