I am fairly certain that there is a header on the cpu board to which a button should be connected.

If it’s AT, there won’t (typically) be a soft power on, it’ll be a mains toggle/latching switch on the PSU itself.

The SBC will likely power up as soon as there are voltage rails present on the backplane.

EDIT: However, there is a 20-pin ATX on the SBC that I missed. In which case there will 100% be a header on the SBC for a momentary switch.

Yes, the CPU board is 20p ATX. Nothing AT there.
I wouldn't be surprised if the pins right of the cf slot, to the edge of the cart, indeed would be for front panel usage. Like LEDs, power, reset...

Looks like an iei Rocky 3786? If so, there's a manual here: https://www.manualslib.com/download/895286/Ie … cky-3786ev.html

Page 33 says CN5 (up near the VGA port) is a 2 pin ATX power switch connection.

snufkin wrote on 2022-05-21, 19:53:

Looks like an iei Rocky 3786? If so, there's a manual here: https://www.manualslib.com/download/895286/Ie … cky-3786ev.html

Page 33 says CN5 (up near the VGA port) is a 2 pin ATX power switch connection.

Will I still be able to use the backplane if the power is coming from the board?

Although I'm having some reservations about using this backplane at all. Finding a case that fits something with that many expansions is almost impossible without paying a shitton (especially in AT spec).

That backplane doesn’t seem to be AT, since the keyboard connector is not on the right place, and as you mentioned, has too many expansion slots.

Indeed, he'll need a case which allows for 14 slots to house that backplane, like the axiomtech AX6156LETB , a 4U 14-slot Rackmount Chassis.

Just to let everyone know the SBC works. I figured out with its dedicated 20-pin ATX connector I could just turn it on by itself without being in a backplane. It's running a 1.2GHz Pentium III CPU and it has 512MB of RAM installed. The CF card slot works and I successfully booted my Windows Me CF card from my other retro PC.

The backplane also seems to work (powering the SBC when mounted on the backplane seems to turn on those LEDs so that's promising -- in the very least the card is communicating through its PICMG slot) but I decided to bail on this backplane and bought a different backplane that's smaller on eBay (a Nexcom NB-9S6P) so I'll be selling off that one.

The plan for all this hardware is to build it around a Voodoo 2 SLi configuration.