I scored a 486 in a full size AT case for $50 at VCF 2024, and it inspired me to go through all my AT x86 gear in the hopes of building up a system. I found the following motherboards in my collection
486 DX 33
This is the motherboard that came with the case. Not winning any speed races for sure. It seems to work OK. There’s some corrosion where the CMOS battery was (which was removed), and I’ve got some CR3032 battery holders on the way to solder in place. Not sure what it’s ultimate fate will be.
486 DX4 100
This I got at a VCF swap-meet last year. It’s a VLB motherboard, and I got a VLB video card & IDE card to go with it. Unfortunately, it appears to be dead. I picked up a POST card and it just shows dashes on the display, and nothing else. Not sure if it’s a dead CPU or bad mobo; I also tried the DX33, but that was before I got the POST card. More investigation required.
Socket 7 w/ Pentium 120
First of all, I completely forgot I had this; I have no idea where it came from, but I found it in one of my retrocomputing boxes. Thanks to The Retro Web’s awesome website, I was able to determine this is a PC Chips M530. Mine says ‘V1.6A’, while the one on the site is ‘V1.6B’. The only visible difference being the presence of a CMOS battery holder, which mine lacks.
And this lack of a CMOS battery holder is proving to be a bit of a pickle. I spent a while going over the documentation and can’t see anywhere this would wire up to. There are a few pins lifted on the realtime clock, a VIA VT82885. It’s a standard DIP, and doesn’t have a battery in it. After looking at a close-up shot I took, I finally noticed a ‘+ ‘ and ‘- ‘ marked above two of the pins.
The pin marked negative is called out as not connected in the data-sheet, but sure enough, giving it 3.3V across these two terminals clears the CMOS battery error and seems to hold the BIOS info. Right now I’ve got it precariously connected to a bench supply; once the battery holders show up, I’ll probably just solder one across these pins. I would like to know what the original arrangement was though. The pins are not only bent out of the way, but are also broken, so I couldn’t reinstate the original config without getting a replacement chip (which a cursory search on eBay didn’t turn up).
There’s a jumper, J7 that’s called ‘External Battery Selector’, and it’s to be left open by default. If closed, I’m just not sure where the external batter would be attached. I suppose I could go probing around to see if any of the jumpers or unlabeled pads land at the Vbat pin on the RTC socket, but without a replacement chip, it’s a moot point.
Is the venerable Dallas RTC chip a drop-in replacement? The pinout of a DS1287 looks similar. I wonder if that’s what used to be in here what I’m seeing is a hack to work around a Dallas chip that had gone bad. For now I think the best course of action (and certainly the least expensive) is to bodge a battery back on to those two pins.
I was able to install DOS 6.22 onto a CF card using a CF to IDE adapter. The card was 8GB, which is recognized by the BIOS, but in DOS it only shows up as 2GB (I seem to remember this… ) and I believe it’s a FAT16 format. So a few questions:
1) How can I multi-boot and have access to the whole 8GB
2) Related, in the BIOS, there are 3 different ways I can set this drive:
- Normal – 15525 Cylinders & 16 Heads
- Large – 7762 Cylinders & 32 Heads
- LBA – 974 Cylinders & 255 Heads
All those options have 16 Sectors. Auto defaults to the same settings as Normal. ‘LBA’ Stands for ‘Large Block Addressing’, but that’s all I remember.
Back to the RTC battery – I got new battery holders, but managed to snap one of the pins trying to solder to it. I even managed to expose some copper with a grinder, but ended up breaking that too. So yeah.. Gonna have to get a new replacement.
According to this Vogons entry from someone who appears to have the same board, the Dallas DS12887A RTC is compatible enough with the VIA82887 / HT12888A. For $6, I can get a replacement from China. For $25, I can get a drop-in replacement board that has a replaceable battery.
This machine is turning into a money pit.
BIOS Update:
I was able to file away a little more of the chip to expose just enough copper to solder to. The key is gluing these copper pads on top of the chip so you can attach to the pins via fine wire and keep any strain from transferring to them from the larger wires.
It “works”!
Jumping around a bit here – here’s what I think the pinout is for the clock display. A collection of pinouts can be found here
Networking:
Thanks to this page, I was able to get the network card working with little fuss
Floppy Drives:
I got a gotek as drive A: & the original 3 1/2″ drive as B:, but I’d like get the 5-1/4″ drive working as well. I found this multi-function card in one of my retrocomputing boxes, and I’d like to enable only the floppy port at first. I’ve found two references to similar cards that use this chip, but not the exact one.
One page suggests this pinout, except I’ve only got 9 pins instead of 11.
This entry in Stason.org lists 12 jumpers, but in both cases, the floppy enable is the first one, so I’m going to assume that’s a pretty safe bet.
No idea whatsoever if this is going to interfere with the built-in controller.
Weeeeell this might be a dead-end. Even with all the jumpers in the disabled position, the machine hangs at the enter setup prompt.
Networking & Soundcard conflicts:
Both the network & soundcard wanted to be at IO 0x220. I could change the IRQ in the SBLASTER line in Autoexec.bat, but changing the IO would shoot an error. Using the network card’s utility, I was able to set it from PnP to Jumperless, and set the IO to 0x240.
With that resolved, I can get the machine to boot with both cards installed, and they both appear to work. They do, however, both share IRQ 5 at the moment. Not sure if that’s going to be a problem. I should also think about a battery hack for the NVRAM on the network card.
SCSI:
I had an Adaptec 2940U kicking around – after a bunch of trial and error, what ultimately worked was calling ASPI8DOS.SYS in Config.sys (following the driver instructions were fine, except for which of the ASPIxxxx files to use).