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My 486 doesn't see different cpu ... can you help me?

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First post, by AlessandroB

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I have an IBM PC330 DX4 which (as the name implies) comes standard with the Intel 486DX4. It is equipped as IBM standard with an additional regulator to bring the 5v to 3.3v.

https://drive.google.com/drive/folders/1-KiNs … y3W?usp=sharing

Here you will find the photos of the panel with the configuration and the photo of the review of the bios, I made the following tests:

1) 486 DX2 66 Mhz: it is seen correctly

2) 486 DX4 100Mhz: it is seen correctly

3) 486 sx 25Mhz: it is seen as 486sx33 (also selecting the jumpers for the 486sx as indicated on the cover)

4) AMD 5x86 P75: the screen remains black with any jumper configuration

5) Cyrix 5x86 GP100: the computer starts but from bios it is seen as an unknown CPU running at 75Mhz

6) Intel POD83Mhz: the computer starts but immediately after checking the ram, when it checks for the presence of a boot floppy, the computer turns off by itself.

Could you comment on the various problems? Thanks

Reply 1 of 6, by TheMobRules

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3) For the CPU to work at 25MHz you need to change the bus speed in addition to set the CPU type to SX, I don't see any option for that, maybe an undocumented jumper?

4) Your BIOS may not support that CPU, and IBM is usually very picky and will not allow you to use unsupported processors. 5x86 use the same settings of an Intel DX4 but with the multiplier set to 2x instead of the default 3x.

5) Cyrix CPUs usually need specific settings, but I'm not really familiar to be honest

6) Make sure J23 and J24 are set to the "P24T" option, and voltage jumper to 5V. Even then, Intel changed POD specs at the last minute, and many boards that claim support for it don't work properly. As a general rule, I've found that the POD only works on boards from 1995 or later.

Reply 3 of 6, by BitWrangler

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Do you feel lucky?
https://www.os2site.com/sw/hardware/ibm/6571/index.html

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 4 of 6, by BitWrangler

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PODs on these need a special interposer it seems, cyrix and AMD unsupported.

edit: ah, there it is... http://ps-2.kev009.com/pcpartnerinfo/ctstips/fbee.htm

Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.

Reply 6 of 6, by AlessandroB

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BitWrangler wrote on 2021-10-06, 01:50:

PODs on these need a special interposer it seems, cyrix and AMD unsupported.

edit: ah, there it is... http://ps-2.kev009.com/pcpartnerinfo/ctstips/fbee.htm

the document explain only how to do with POD, but nothing with cyrix and Amd 5x86.... any suggested test?

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