The 486 Restoration – Part 3

Welcome to part three of my 486 restoration project! Check out part one and two for more information on the parts I rescued from a badly neglected machine. I’m happy to report that the purchase of this banged up machine was not in vain. It didn’t come without it’s share of challenges but as you’ll see in this installment – it’s alive!

After removing the barrel battery and constructing an external battery pack in part 2, the next order of business was to get the machine put together on the work bench and powered up.

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My test-bench isn’t pretty but it’s functional!

I’m using a modern PFC Seasonic 350W power supply with an AT 12-pin adapter. These old systems run almost entirely on 5 volt power and draw nothing from the 3.3V and little from the +12V rails. This can cause problems with some newer PSUs, but this Seasonic model fairs well with a 130W rating on the 5V rail. The only side effect of this power draw imbalance is a higher than usual +12.6V on the 12V rail. It’s not ideal, but I’d rather this than a flaky 25 year old AT power supply.

Since the system didn’t come with a video card, I pulled out an old ATI Mach 32 ISA card from the parts bin.

 

I recently picked this up from the great folks in the computer recycling department of The Working Center in Kitchener. It was sitting in a box full of old PCI graphics cards destined for e-waste. It’s always awesome to keep classic parts out of the landfill and support a great cause at the same time.

Although ISA video cards are bandwidth starved due to the 8MHz bus speed, this would do in a pinch until I could find a VESA local bus card. You’ll notice one interesting – but pretty useless – feature on this card. I first thought it was some kind of S-video connector, but turns out it’s actually a ‘bus mouse’ port.

For storage, I’m using a handy compact flash to IDE adapter that I discussed in detail here.

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High quality, SLC flash makes this a robust IDE solution. 4GB is more than I need.

The card is an industrial grade 4GB card made by Cactus Technologies. It’s the 303 series, which uses high quality SLC flash and is designed for this type of use case. With a 35MB/s rating and SLC flash it performs well and will survive a large number of write-cycles. I’ve had issues with second hand MLC consumer grade cards in the past so it’s nice to have some peace of mind.

 

I was happy to see that the system posts without issue and that time/date and all BIOS settings are retained with the new CMOS battery pack connected.

The Frustration Begins

Well, it wouldn’t be a classic PC build without some frustration and heartache. Despite the small success of getting the system powered up, I continually ran into problems with the Pine PT-627A I/O card. It seemed that whenever I tried to boot with a floppy drive configured, I’d get a FDD controller failure.

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I had similar issues with the IDE controllers. Whenever I would configure C/H/S settings or do an auto-detection, the BIOS would report an IDE controller failure also.

This problem was a real test of patience and was almost comical from a troubleshooting perspective. I was initially convinced it was a resource conflict – possibly due to the bus mouse controller on the ATI card. I literally spent hours trying different combinations of enabling/disabling components on the card as well as different I/O addresses and IRQ assignments. Strangely, I would change the jumpers on the card and it would start working. Then I would connect a cable or change a jumper and it would stop again. Then if I put it back the way it was when working previously, it refused to work.

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R.I.P. Pine PT-627A. I’m sure you had many good years of functional I/O connectivity.

At one point I had it working – probably with jumper settings I had already tried – and it detected the IDE and floppy drives. I thought I was off to the races and was in the process of formatting the hard drive when I noticed the IDE cable wasn’t fully seated on the pins. As soon as I touched the card with even a slight bit of pressure the DOS format tool started spewing tons of errors. Upon reboot, the same controller failure happened again.
The card must have a flaky trace or component that would intermittently connect/disconnect with any pressure applied to the connectors or jumper banks. So, in the end, it wasn’t the resources or jumper settings – it was just the action of applying pressure to certain parts of the card.

After close inspection, thorough cleaning with isopropyl alcohol and my best efforts, it eventually stopped working altogether. Nothing would rouse it out of this state. Rest in peace Pine PT-627A.

VESA Local Bus Video and I/O Card

I was pretty disappointed about the Pine PT-627A, as this system is essentially useless without an I/O card. My initial eBay searches were not very promising as VLB cards are becoming quite expensive. In fact, most of the VLB I/O cards I saw were priced higher than what I paid for this entire system. I was planning to spend some money on a VLB video card, but really wasn’t counting on having to buy a new I/O card to go with it. That’s when I came across a real gem – from a Canadian seller too to my surprise.

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VGA and I/O functionality in one card!

For less than the price of most I/O cards, I was able to find this combo VLB VGA and I/O controller card. Talk about the best of both worlds – VESA graphics and the I/O functionality I needed.

I couldn’t find much information on this specific model, but it’s a Trident 9400 VGA chipset with 1MB of video memory combined with several I/O chips on the same card. I believe that the VGA and IDE controllers are both on the VESA local bus, and that the I/O components for serial/parallel and floppy use the slower ISA bus. If I’m not mistaken, the IDE controller is the Acer M5105 and I/O is provided by the ALI chip closer to the slot bracket.

 

There are sockets on the card for additional video memory, which is a plus. The 1MB onboard should be plenty for my purposes though as I’m not concerned about higher resolutions.

About the only thing this card doesn’t have that the Pine PT-627A did is a second IDE controller channel. I didn’t really care though – it’s unlikely that the BIOS would support an ATAPI CD-ROM drive anyway, so a single IDE channel is fine. I could always connect a second drive as a slave device if needed.

 

The back of the card provides good notes on the jumper options. Notice that the card has a jumper for >33MHz FSB operation as well, which could be useful. After disabling a few ports, including the second serial port and the parallel port, I got it setup on the bench. I couldn’t find a way to disable the game/joystick port, unfortunately.

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It’s not a small card, but having everything integrated makes things neater.

It’s Alive!

Gotta love the colorful Trident VGA BIOS. My first 486 system back in 1994 also had a Trident card so this was a bit nostalgic and caught my eye.

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To my surprise, the BIOS detected the 4GB compact flash card without issue. Despite this, the board does still have the infamous 504MB limitation. This is expected and can be easily worked around with drive overlay software.

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After getting DOS installed, I copied over Phil’s DOS Benchmark and some games to try out.

 

Everything was very stable. I’ll probably do some further tweaking of DRAM wait states when I get some time, but it performs as expected.

 

What I was most pleased with was the graphics performance. Gaming frame rates were nice and smooth compared to my other DEC 486 system with ISA based graphics. VESA local bus really is that much better when it comes to DOS gaming. If I get a chance later I may try to do some comparison benchmarks.

What’s Next?

Next up, I’ll need to find a period-correct ISA sound card to use in this machine, as well as a CD-ROM drive. I’ll also need to find some kind of a suitable case for this, or somehow restore the wasted one. Stay tuned for more!

<<  The 486 Restoration – Part 2

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