My 286 Mainboard build \ VOGONS

My 286 Mainboard build

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First post, by srmeister

Posted on 2022-09-17, 11:12

srmeister Offline

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Hi,
currentyl i am developing a 286 Mainboard, because i thought it would be fun, plus they seem to get more and more rare.
Tried to use new components as much as possible. The CPU is original. Right now i am using Intersil / Harris 286 at 25Mhz.

System specs:
- uses original unpatched IBM AT BIOS (circumvents speed check by changing timer speed while booting). i plan to support other generic 286 BIOS but its untested as of now.
- SRAM instead of DRAM, just for ease of use. Right now its 4MB at 10ns so zero waitstates.
- yes, chipset is implemented in FPGA.
- has 7 ISA Slots, frequency at 8,33 Mhz but can be changed.
- can use normal PS2 Keyboard (or AT Keyboard via cable adapter)
- option to use PS2 Mouse
- option to use microSD card as HDD
- BIOS and option ROM (XTIDE Universal Bios in this case) can be loaded via USB cable and are shadowed into RAM. I plan on option to shadow Video BIOS ROM too.
- can use ATX or AT Power supply and option to add negative 5V regulator
- can be installed in AT or ATX case. Keyboard connector is at the usual place for AT case.
Right now added ISA Cards are: Trident TVGA 8900D VGA card and generic Multi-IO card.

I would like to hear your opinion on it.

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Reply 1 of 10, by srmeister

Posted on 2022-09-17, 11:14

srmeister Offline

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some more pictures.

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Reply 2 of 10, by maxtherabbit

Posted on 2022-09-17, 12:59

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Awesome! Nothing like a 0WS 25MHz 286

Reply 3 of 10, by pshipkov

Posted on 2022-09-17, 20:17

pshipkov Offline

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If the synthetic tests are to be trusted - this must be the fastest clock-to-clock 286 implementation to date.
Great job !

retro bits and bytes

Reply 4 of 10, by jakethompson1

Posted on 2022-09-17, 20:32

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Excellent. I have a homebrew8088.com board, this seems in the same kind of spirit but interesting to see what you had to overcome via an FPGA chipset. I wish I could do that kind of stuff.
How much different would it need to be for an 8086 or 386SX?
Someday someone will take on a 486 VLB board... and bypass the whole slot issue by just putting one of the ultimate VGA VLB chips right on the board.

Reply 5 of 10, by Nexxen

Posted on 2022-09-17, 23:20

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I dream of this and someone just make it happen.
Talk about luck.

Last edited by Nexxen on 2022-09-18, 09:07. Edited 1 time in total.

PC#1 Pentium 233 MMX - 98SE
PC#2 PIII-1Ghz - 98SE/W2K

Reply 6 of 10, by maxtherabbit

Posted on 2022-09-18, 01:54

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How about adding a mini-DIN for the mouse as well below the keyboard connector? some late AT boards did this and it would obviously still be usable in an ATX case with a custom IO shield

Reply 7 of 10, by ajacocks

Posted on 2022-09-18, 02:43

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This is very impressive! I'd love to build one of these, if you decide that you want to sell or share the design.

- Alex

Reply 8 of 10, by Anonymous Coward

Posted on 2022-09-18, 03:27

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Does this board support UMB and LIM 4.0 in hardware?

"Will the highways on the internets become more few?" -Gee Dubya
V'Ger XT|Upgraded AT|Ultimate 386|Super VL/EISA 486|SMP VL/EISA Pentium

Reply 9 of 10, by srmeister

Posted on 2022-09-18, 07:09

srmeister Offline

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Hi,

pshipkov wrote on 2022-09-17, 20:17:

If the synthetic tests are to be trusted - this must be the fastest clock-to-clock 286 implementation to date.
Great job !

I think it is the fastest. Not sure but i read that usually at 25 Mhz, zero WS is impossible with normal RAM.

jakethompson1 wrote on 2022-09-17, 20:32:

How much different would it need to be for an 8086 or 386SX?

If this will be finished, i think i might try a 386 build, but the problem may be to get sockets. (same as for 486) maybe one would need to solder the CPU which i dont really like, expecially on 486.

maxtherabbit wrote on 2022-09-18, 01:54:

How about adding a mini-DIN for the mouse as well below the keyboard connector? some late AT boards did this and it would obviously still be usable in an ATX case with a custom IO shield

Yes this is one thing i still want to change. but i also want to keep the option to use pin header for mouse so it can be placed anywhere.

ajacocks wrote on 2022-09-18, 02:43:

This is very impressive! I'd love to build one of these, if you decide that you want to sell or share the design.

- Alex

It isnt finished and i didnt yet think about what to do with it once its finished. Maybe i will run a small batch of this. The most expensive thing, if you only did one board, would be the PCB at around 250€ plus all the parts and you reach 400-500, but, in a batch, it would be much more affordable.
Problem is that the board is 6 Layers and this is not cheap 🙁

Anonymous Coward wrote on 2022-09-18, 03:27:

Does this board support UMB and LIM 4.0 in hardware?

Well, i guess UMB is supported. The RAM in this area is available, also it is used to shadow the ROMs. Regarding extended memory: Gate A20 works like in IBM AT. But i am having some issues with himem.sys, as it shows memory errors on boot, when every existing memtest program works for hours and doesnt find any errors. I would be happy to get any suggestions on this.

Also, there are some things still missing. DMA isnt working yet, so no Soundblaster and Floppy drive 🙁

Will keep you updated. And feel free to ask any questions.

Reply 10 of 10, by pshipkov

Posted on 2022-09-18, 17:21

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I was actually able to achieve 30MHz with 0-wait states on 2-3 motherboards with curated RAM SIMMs.
That is fully stable.

retro bits and bytes

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