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Need help building Pentium MMX VRM module (lots of information inside!) (re: IBM PC 330/350)

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First post, by sofakng

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The IBM PC 330/350 models offered CPUs up to Pentium MMX 233 but they excluded the 2.8v VRM module unless it came with an MMX CPU. However, the pin headers are still present on the motherboard and the BIOS supports it without any issues.

The IBM part number (FRU) is 76H3658 which corresponds to a Semtech MP55C-3.3-2.8 VRM (datasheet).

The VRM module appears to be a simple 30-pin adapter with a 5-pin voltage regulator and a few resistors/capacitors:

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I've compared the Semtech datasheet and used a multimeter to diagram the pins on the IBM motherboard:

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This is about as far as I can go myself so I'm hoping somebody with more knowledge can help out?

If somebody could help me with the VRM circuit I should be able to design and order some PCBs and distribute them to everybody interested.

Thanks so much for any help!

Reply 1 of 3, by BitWrangler

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If we were in an ideal world, you could just hang a module like this on the 12V line, though I'd do it through a drive connector, and feed it's output to the core voltage pads. https://www.amazon.ca/WHDTS-1-2V-35V-Converte … /dp/B0819Y8PJZ/
However, we don't have knowledge of how the presence detection works and whether it's automatically switched. Also there's no information about a jumper change to change from dual core to split core. You should look for that, and confirm you get zero or very low parasitic voltage on the core pads when set.

It might need some smarts though, that chip on there looks annoyingly like one of those OTP 4 bit CPUs.

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 2 of 3, by sofakng

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Thanks for the reply!

Yeah, I'm sure it will be more complicated than it seems, right?

Are you talking about the SENSE/DISABLE pins on the regulator module or something different? ...or are you talking about something on the motherboard to detect the regulator module and enable it?

There doesn't appear to be any pins or jumpers to configure. There are only one set of jumpers to select CPU speed but curiously it only uses 4 of the 6 jumper pins and the other two are not documented.

The BIOS seems to somewhat support the chip though. It displays "Pentium MMX" correctly but it displays "100 MHz" (jumper setting) when it's actually running at 233 MHz.

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