Tuesday, August 22, 2006

IBM PS/2 Model 25 used as a serial console

Manufacturer: IBM
Model: PS/2 8525
(a.k.a. PS/2 Model 25)
CPU: 8086, 8MHz
RAM: 256kB Base, 768K loaded
HDD: None
Release Date: Aug 11, 1987

The Personal System/2 was one of IBM's longest lasting desktop product lines. Although USB has recently become the more popular human input device (keyboard/mouse) interface, the IBM PS/2 computer is the namesake of the older PS/2 keyboard and mouse ports found on most IBM PC Compatables in the 90's and through today. The 8525 was one of the first machines to use 72-pin SIMM memory modules instead of the older 30-pin SIMMs. the 72-pin layout would also set a long-running standard in the computer industry, lasting well into the late 1990s on many computers even beyond the PC/XT compatible realm (Macs, Sun, RS/6000 and others)

The PS/2 8525 borrowed the "all in one" design from Apple Computer, placing the monitor, motherboard, internal peripherals and drives into one case. The end result was a machine that's about as bulky as my Performa 550 but the same age as the Macintosh SE. The screen is much larger, and the later revisions of the 8525 offered a 16 color MCGA display. The first batch shipped with monochrome monitors.

Inside, the MicroChannel Architecture motherboard and riser card provided 2 MCA expansion slots. The most common drive configuration was two 720k 3.5" floppy drives, however a spendy option package replaced one of them with a miniscule (by today's standards) internal hard drive.

Ultimately, the 8525 found its role in education, much like Apple computers of similar vintage. A few companies dared to offer upgrades targeted just for this family of machines. IBM itself eventually released a 286-powered version, and from there, companies begun selling 386 and 486 upgrades, some of which involed completely swapping motherboards. As this system had built-in video and a small internal form factor, there was no hope for simply buying another generic motherboard. As a matter of fact, the "generic motherboard" and "build your own PC" concepts wouldn't show up until several years after the 8525 was announced.

My last weekend edition post talked about thin clients, and even mentioned "green screen" terminals of days gone by. This system has pretty much been a "thin client" of sorts ever since I purchased it in 1995. For the longest time, it was used only as a BBS terminal. Paired with MS-DOS and TeleMate, a modem/terminal emulator program, my girlfriend (now wife) and I would use it to connect to BBSs, MUDs, UNIX servers and other dial-up resources.

Later on, I used telemate in a less traditional role to turn the 8525 into a text terminal for my Linux box, in essence, it was a very primitive thin client, used for reading my e-mail, writing for my old e-zine, and doing remote system administration work.


John Kelly said...

Just out of curiosity, have you got diassembly instructions for the IBM Personal System/2 Model 25? My mom has had one for years and it recently started sparking on start up. I can see through the vent holes where it is sparking, but can't get the case open.

Noah said...

It's been ages and this dinosaur is buried under a bunch of other things right now. To the best of my recollection, two screws on the lower back are removed, and the main chassis is laid screen-down, then you hinge the bottom and lower-rear panel down. There may be screws on the bottom as well. This should help.

Nickname unavailable said...

A few corrections.

The original 8086 Model 25 used 30-pin SIMMs, not 72-pin. (The 286 and 386SX models used 72-pin.)

All Model 25's used the old PC "ISA" bus, not Microchannel. (The 8086 models used the original 8-bit PC version, the 286 and 386SX models used 16-bit AT version.) The 25, and it's monitor-less sibling, the 30, are the only PS/2 to use ISA, the rest all use Microchannel.

The 256-color MCGA version came out at the same time as the grayscale model. The grayscale (which is actually black and white, not green,) version has the same MCGA controller, though.

While it could physically take 768 KB of RAM, it could only address 640 KB. (In fact, to get to max addressable 640 KB, you had to install 768 KB!)

Yes, IBM made a 386SX version. And there were many 286-to-386 upgrades that ONLY replaced the CPU. (I had one back in the early '90s.)

Anonymous said...

Umm I have a problem, I just got a IBM PS/2 Model 25 and there is something wrong with the monitor, it will start up but the monitor will just be lines moving down the screen or just a box in the middle. It has no HD. But it does have a floppy.
Do you know how I can fix this?

Noah said...

could be anything from a loose connector on the back of the CRT (be careful in there. High voltage. Achtung. etc.) to a bad video controller or failing capacitors. I've never seen that on one of these all-in-one PS/2's before, but similar stuff happens for reasons mentioned above on Mac Classic and SE/30s.

Anonymous said...

Well thank you anyway, and I just figured out the ram is missing so I might not even bother with this computer, just put it on display