ASUS ISA-486 motherboard missing support for dx2/66? \ VOGONS

ASUS ISA-486 motherboard missing support for dx2/66?

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First post, by perhenden

Posted on 2021-04-05, 12:33

perhenden Offline

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I've got this Asus ISA-486 motherboard [1] with an Intel 486 DX2/66 CPU.
It only registers as 50MHz [2], even though I've set the jumpers for 33MHz (x2).
The CPU is labeled as 66MHz, and another 66MHz CPU also register as 50, even though they appear as 66 on another motherboard.

Specifically I've set J2=2-3 and J3=1-2, following the jumper manual at
[http://www.win3x.org/uh19/motherboard/show/806]
The turbo switch is set to 'enabled' (closed).

Is there something I can do to make it run at 33MHz base? There is only a jumper setting that says 25/33, not a specific one for 33.

[1] Revision 1.4. Bios version 1.0.2 (05/05/91). Bios string 40-0102-001292-00101111-050591-SIS-486-F.
[2] CHKCPU, HWInfo and QAPlus all report the CPU as 50 MHz, at 2x25MHz

Reply 1 of 2, by vetz

Posted on 2021-04-05, 12:37

vetz Offline

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You most likely need to swap out the clock crystal from a 25mhz one to 33.33mhz. It's the part on the top right corner that has a white plastic ziptie.

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Reply 2 of 2, by perhenden

Posted on 2021-04-05, 13:40

perhenden Offline

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Location: Oslo, Norway

That sounds correct. I do have a 25MHz one.
It's labeled "Koyo KCO-010T 25.000MHz 91-22". A 4-pin DIP oscillator, it appears.

What's a good replacement? I see some original parts from "sourcing agents", but that'll be expensive for just one item.
Would 32.768 or 32 MHz ones also work? Those seem more common today.

EDIT: Found one on eBay, [https://www.ebay.co.uk/itm/IQXO-350C-33-333MH … or/303558319181]. Comparing specs I see that pins and size for the IQXO-350C series matches the board. It's slightly broader (less than 1 mm difference) than the original, but there's enough room on the board. The MXO45 series also look compatible.

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