Nexxen wrote on 2025-09-06, 13:27:

Could the HT12 be replaced by some "pie", or other project?
They aren't mainstream to find... just curious.

Might be tough to emulate a 20 Mhz memory interface and ISA bus with a Pie.

But if you are going ATX, I guess you could include a Picogus to fill out the Sound and USB ports on the standard ATX IO shield. How cool would that be? 286 with a USB port that can do CD emulation?

Hello Nelson68k !

This is a really good project ! And I happen to have the original board.
I hope this isn't too off-topic, but basically my board came with a 12MHz 286 and when I saw that they were also made with 20MHz CPUs, I couldn't resist and I installed a 20MHz harris 286 (to clarify because someone will probably ask, it doesn't look fake to me at all, it has a datecode from 1993 and it doesn't look like chip that got painted), but the board is not quite stable anymore after I installed a 40Mhz crystal ...

It POSTs and counts RAM, but it usually hangs after that. If I disable turbo, running the CPU slower, I can sometimes get in the CMOS settings, but there's nothing I can adjust there.
I saw in your changelog on github that you wrote "V0.5 Stabel System at 20Mhz with no RAM errors 😀".
Is there anything special you made to make it stable for 20MHz other than PCB rework ?

Also, my board came with a HT12 chipset, not the HT12P/16A, so I'm wondering if I'm just running an older chipset who's just not stable at that speed ...

Lastly, when I got my board the KBC was missing, so I put a new one I grabbed from a dead 386SX board. The reason I mention this is because when it hangs, the board is still alive to some extent, but pressing keys triggers angry "bleeps" and when I checked the chipset's datasheet and also verified on my board, I noticed that the chipset is made to take two separate clocks for the CPU and ISA bus, except that on this board, the two pins are joint.
This causes the ISA bus to run at 10MHz when a 20MHz CPU is installed which is a little bit high ... So maybe I have a speed sensitive KBC ...

Multiple variables I know 😅

TL:DR I upgraded my board which hangs on POST, and I'm suspecting
- that I have to change something else on my board
- that my chipset is just a revision too old
- that my replacement KBC is speed sensitive

(also where did you source those HT12P/16A ? If my chipset happens to be too old, I'd gladly replace it with a newer one 😁 )

286-386 KBC (software based) are sketchy above 45Mhz system bus. At 20Mhz it should be fine...

I think the older chipset might be limited to 12MHz..

Hi@Deksor, I also think it's the chipset. The HT12/16A is rated for up to 16 MHz. So 20 MHz is possible. If you have an older 12 MHz chipset, 20 MHz is obviously too much. I'm familiar with the behavior you described when I go beyond a 24 MHz clock speed.
That dealer no longer exists. But a chip broker might still have some. I can send you a 16 MHz chipset if that helps.

Hi,

I saw you bought a tray of HT12 chipsets for this project. Would you by any chance have a spare one you'd be willing to sell?

My Octek Fox II 286 died after I plugged in a homemade AdLib replica with a Gerber error, tiny vias shorted the A and B sides of the ISA slot, putting +12V on SD0 (and -12V/-5V on SD2/SD4). The HT12's SD0 pin now reads abnormally in diode mode, the POST card only shows odd codes (bit 0 stuck high), and the board halts with 3 beeps (base 64K memory failure). RAM is fine in another system.

I'd be happy to pay for the chip and shipping (I'm in France). Thanks a lot!

douglar wrote on 2024-12-17, 21:10:

That's an exciting looking board. I have a pair of 45ns 30 pin simms that I've been saving for a board like that.

Makes me wonder what else could be done to make an uber 286 board.

A PS/2 mouse header? A PLL for selecting clock speeds? An OPTi 82C281 build that has a cache ?

Cache would be interesting for 25mhz+ operations, especially the “chipset cache” chipsets.

Compliant ps/2 onboard could save a slot.

Certain late model OEM 386sx systems (like Compaq) had onboard components that were local bus operating at full bus speeds (video/hd) which might also be interesting since you could save a slot and reduce video latency

Matt5377 wrote on 2026-10-16, 14:40:

Would you by any chance have a spare one you'd be willing to sell?

Hi, send me a PM.

Hi, Nelson68k,
Your FOSS project on this motherboard replica is great just like the M396F replica project.

I noticed, that you used some adjustable frequency generator on this photo:

Could you explain exactly what this small PCB with the tiny chip and the crystal oscillator is? Thanks!

analog_programmer wrote on 2026-10-20, 08:21:

Hi, Nelson68k, Your FOSS project on this motherboard replica is great just like the M396F replica project. […]
Show full quote

Hi, Nelson68k,
Your FOSS project on this motherboard replica is great just like the M396F replica project.

I noticed, that you used some adjustable frequency generator on this photo:

Could you explain exactly what this small PCB with the tiny chip and the crystal oscillator is? Thanks!

https://www.youtube.com/watch?v=odcH80wiiPs

Nexxen wrote on 2026-10-20, 10:34:

https://www.youtube.com/watch?v=odcH80wiiPs

Do you know what PLL chip is used?

analog_programmer wrote on 2026-10-20, 11:05:

Do you know what PLL chip is used?

https://migron-electronics.com/assets/docs/an … -clk-manual.pdf https://migron-electronics.com/product.html?id=MIG-CLK-006
points at $2 ICS501 https://www.renesas.com/en/document/dst/501-datasheet
actually multipliers match ICS511
$1 each if you buy 10 of slightly revised https://www.renesas.com/en/document/dst/511-datasheet
bonus points for getting the 501MILF 511MILF versions 😀

rasz_pl wrote on 2026-10-20, 12:21:

points at $2 ICS501 https://www.renesas.com/en/document/dst/501-datasheet
$1 each if you buy 10 of slightly revised https://www.renesas.com/en/document/dst/511-datasheet
bonus points for getting the 501MILF 511MILF versions 😀

Thank you very much, rasz_pl! Perhaps these are not the exact chips used in this simple device, since they do not cover the input crystal frequency range of 2 - 33 MHz, but close enough 😀

This migron PCB has the part number rubbed off. However, I was able to use NB3N502DG and 512MILF for a similar design. I made a few of these with just a protoboard. See this thread: 386 Clock Generator Replacement

feipoa wrote on 2026-10-22, 15:43:

I made a few of these with just a protoboard. See this thread: 386 Clock Generator Replacement

Nice work. Thanks.