I had a Tandon once (I actually kept the box long after I let it go because it was the perfect size to keep my Altair (very vintage) in)

I don't recall these details but I do think it was fairly "normal" for the era.

Looking at your pic - the power connector would be is the 12 inline pins behind the LPT port) - 12 counting the "hole" where a pin is missing (key)

This is most likely "AT" (not ATX) - the rounded pins seem a little off - I recall AT being 12 inline "flat" pins with two 6-pin connectors from the power supply that fit the 12-pin mainboard connettor by placing them end-to-end - you always put the black wires (gound) together at the center - I think I might have had at least one AT system with rounder power pins at one time.

It also has a 5-pin D.I.N. keyboard which would also place it in the right era for n "AT" supply.

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You should be able to see if the 4 center pins (two for each power cable from the PSU) are connected together and use a multimeter to confirm that this is ground.

If so look up "AT power connector pinout" and you should be able to find what is most likely correct.

It's a good thing you have the rizer - not only because they are hard to find now, but also because the ISA bus has direct connections to the +/-5v and +/-12v power rails, and you can confirm that these go to the correct pins for an AT supply pinout.

That looks only slightly less weird than the Tandon Targa/20 AT I used to have. The power supply was proprietary as was the very stylish case.

Many Bothans wrote on 2026-07-27, 22:00:

That looks only slightly less weird than the Tandon Targa/20 AT I used to have. The power supply was proprietary as was the very stylish case.

Even if it is a proprietary pinout (many "proprietary" PSUs at least followed IBM-AT pinout), you should still be able to work out the actual pinout by referencing connector pins back to the ISA power "fingers".

The 386sx piggyback module looks like a drop in replacement for a 286.

eisapc wrote on 2026-07-28, 10:46:

The 386sx piggyback module looks like a drop in replacement for a 286.

Agreed - it would be interesting to see if there's some sort of special mount socket adapter to male it fit a 286 socket or if the board was actually designed to accommodate either (there's a lot of extra "stuff" on the adapter card which suggests it might be adapting to a different set of interface signals)

Thanks guys for your help. I'll first try to check continuity between ground and interior sticks then the connection to ISA slot.
It's true that it's more of a 286 motherboard with a 386SX daughterboard.