Getting a non-functional IBM PC/AT up and running is hard

 It's been a long time without posts. Maybe I can correct this.

A couple months ago, I took the plunge and got an IBM PC/AT. I've wanted one for almost 40 years. I have an IBM PC, two PC/XTs, and a 5160 portable. A 5170 is next in line ATs were very hard to get for the last couple years but units are showing up on eBay with various descriptions, usually untested. No keyboard, monitor, cables, etc. So one goes off a few pictures and hopes for the best.

Here is the one I got late 2022. I knew it had two floppies, some type of display card, a hard drive, a memory add-on card and two network cards. The seller threw in a non-IBM (cheapo) keyboard which was great. I got it on the bench and start to look. The Seagate ST-225 drive is unresponsive, it doesn't spin - a bad sign. It has a generic IBM Monochrome card, ok. I splurge for an ATI Ultra ISA VGA card with bus mouse off eBay. That'll allow me to use a VGA flat panel monitor.

I quickly learn that the PC/AT is not like the earlier IBMs. It keeps BIOS configuration information in CMOS rather than via motherboard jumpers. And this memory can only (mostly) be set via a special IBM 5.25" diagnostics floppy disk. If you have this problem, see this website on how to proceed. I got the disk image and took it to my Pentium III PC and wrote a disk. I did use a 1.2MB drive to write a 360K disk as my 360K drives were misbehaving. It worked, I could set parameters in the AT by booting the diagnostics disk.

I replace the BIOS memory back up battery (not corroded thank goodness) with an AA battery pack. All good. See this on the procedure.

With the diagnostics disk, I could finally boot, displaying on the monitor. I learned the AT is a model 339, 512KB memory onboard (not stacked DIP chips). Drive A works, floppy B doesn't (seek errors), and the hard drive is indeed dead.

Here are the add-in cards (besides the floppy/hard disk card not shown). From the top, an IBM monochrome adapter (plus serial/parallel), 10-base-T network card, Serial/parallel card, and an interesting IBM Token Ring network card. None immediately needed to debug/use the machine but I'm not complaining. 

I get an XT-IDE ISA to compact flash card and put in a 16MB CF card. I had issues with that until I used the XT CD-ROM in the Pentium III to mark the drive via FDISK /MBR. Then I could format the drive and I loaded PCDOS 2. I upgraded it via floppy to MS-DOS 5.0. There are a slew of XT-IDE cards out there and some don't play nice with the AT without clone ROMs which would be a huge headache. So I'm glad the one I have played ok.

Meanwhile, I didn't turn my alert off on eBay and another IBM PC/AT machine popped up at a good price. Amy bought it for me for a birthday present. It has parts that complement the PC above. I'll dive into that one in the next post.