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Problem with I/O controller in a small retro PC project

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First post, by pvlst

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I want to build this small-sized retro PC. I was able to get the 4-Dimension branded case that I remember from my childhood and I want to use this case for the PC. However, the case is quite small, it uses a proprietary power supply and it has to use an ISA riser of a rather uncommon format. The riser contains only 2x 8bit and 2x 16 bit ISA slots. It can't be extended, i.e. soldering 16bit slots, I'd have to make my own riser which I'm not capable of doing (the last time I worked with PCB design software was about 20 years ago).

The attachment 4dpc_1.jpg is no longer available

I want the PC to have
- 16bit ISA VGA card (no problem, various possibilities, I'll probably want to use a Tseng ET4000)
- Sound card (no preference, but I have this little 8bit ISA AZTech sound card that seems like a good option)
- Network card (I have the original non-B 3C509, which seems fine and should also work in 8bit ISA)
- COM (or PS/2) port for mouse + CF card as harddrive + floppy drive (now this is where it gets complicated)

I'm really struggling to put all this together while having available and working the I/O ports mentioned above. Due to the design of the case, the side of the riser facing the upper part of the mainboard (towards fan in the picture below) contains only 8bit ISA slots, where there is really little space around them, and also, one of them is limited to COM and LPT sized back headers. See these pics:

The attachment 4dpc_2.jpg is no longer available
The attachment 4dpc_3.jpg is no longer available

So the main problem is the I/O controller.
There doesn't seem to be any 8bit ISA card with COM port and 40pin IDE that would be bootable. I tried 3 different 16bit multi I/O cards in the 8bit slot, but none of them would boot. When I moved them to the 16bit ISA slot, they all booted fine.

I'm considering using a custom-made low profile ISA card adapter with soldered cables, similar to the PCIe risers that are widely available these days. I'd be able to connect small XTIDE card to it and mount it somewhere in the case. The COM port (and LPT) would be provided by a small 8bit ISA I/O controller that I do have. The only thing missing is the floppy port. Ideally, I'd like to have it on the 8bit ISA I/O controller, but although there is s floppy+COM made by Sergey Kiselev, it doesn't have the COM port in the typical position, so I wouldn't be able to mount it in the case. I haven't found anything usable with COM(+LPT)+floppy.

I checked this adapter, it seems very nice, but it doesn't have a COM port:
https://www.knaapic.nl/wordpress/8-bit-ide-fl … pt-adapter-card

I also checked the XTIDE options, again no floppy/COM port though:
https://minuszerodegrees.net/xtide/variations … 0variations.htm

Do you have any other recommendations on how to achieve this floppy+COM(+LPT)+IDE combo under such conditions? I was considering using a VGA/sound card with IDE headers, but I read that they are usually not bootable.

I'll post more about the project later.

Reply 1 of 3, by dionb

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In a similarly constrained system (actually worse, I only have two slots) my solution was to go for a VGA+multi I/O combi card in 16b I/O, freeing up my other (8b) slot for an 8b Aztech SB clone. Those combo cards are uncommon, but tend to be unloved, so when they do pop up they're no more expensive than a similar VGA-only card. I've seen combo cards like this with Cirrus Logic GD542x chipsets and with UMC 85C4xx chipsets. I have VLB cards with both but my ISA card had a UMC chipset. For Windows it's a slug, but in DOS it's not better or worse than other ISA chipsets, completely bottlenecked by the bus of course.

Reply 2 of 3, by pvlst

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dionb wrote on 2025-03-16, 19:07:

In a similarly constrained system (actually worse, I only have two slots) my solution was to go for a VGA+multi I/O combi card in 16b I/O, freeing up my other (8b) slot for an 8b Aztech SB clone. Those combo cards are uncommon, but tend to be unloved, so when they do pop up they're no more expensive than a similar VGA-only card. I've seen combo cards like this with Cirrus Logic GD542x chipsets and with UMC 85C4xx chipsets. I have VLB cards with both but my ISA card had a UMC chipset. For Windows it's a slug, but in DOS it's not better or worse than other ISA chipsets, completely bottlenecked by the bus of course.

So far I've only seen this one for ISA on TRW: https://theretroweb.com/expansioncards/s/letex-mio-a
And it's OTI chip based, which makes me think that the performance would be unacceptably bad even for DOS. I can't find anything similar anywhere on eBay or anywhere else, though.

Reply 3 of 3, by dionb

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pvlst wrote on 2025-03-16, 19:28:

[...]

So far I've only seen this one for ISA on TRW: https://theretroweb.com/expansioncards/s/letex-mio-a
And it's OTI chip based, which makes me think that the performance would be unacceptably bad even for DOS. I can't find anything similar anywhere on eBay or anywhere else, though.

They're uncommon, so some patience might be needed. There's no technical reason for them to have a particular chipset, it's just two different ISA devices on one card (well, more actually for I/O, that's usually at least three separate ones on its own). I found mine after about 6 months of keeping an eye out for something similar on the local Craigslist equivalent. Can't find a pic of it anywhere, it's in our little retro system at work.

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