I was looking at a similar thing recently, but I believe the backplane itself has no "logic" as such, so the card with the CPU on it drives the ISA/PCI bus (generates the clock and other signals that get sent out to the bus, and picked up by the other cards.) If you were to plug two of the cards into the same backplane, they'd both be sending out clock signals which would conflict, and neither one would work.

It's a nice idea in theory, but apparently these systems were only ever designed to have one "computer card" installed. I think the idea was that you could swap out a 386 card for a 486 card and the machine would be upgraded without needing a reinstall or changing disks, etc.

computergeek92 wrote:

Can you have multiple single board computers in one backplane system? That would make a one of a kind multi system. 😉

I think this would cause a whole pile of problems - address conflicts, etc. Even if you didn't have any other ISA cards plugged in the SBCs would be duplicating each other's IDE controllers, serial ports, etc. over the ISA bus. Never tried it myself though so take that with a grain of salt.

It would be neat if there were a way to switch between them though. My DOS PC is an Advantech P233MMX SBC on a massive 14-slot ISA/PCI backplane. I'd love to be able to select between, say, that, a P3, or a 386DX as 'alternate configurations' and let them use the same expansion cards.

Maybe you could add a switch that cuts power to a couple of the slots, so only one SBC is powered up at a time?

That would drag down the bus. The inactive board would have to completely switch off its signals to the bus. This would not even happen when keeping one particular board in reset. You'd have to check which lines are not cut in reset and add additional logic.

You need a dumb backplane that only supplies voltages to the slots and nothing or else as previously stated the boards will conflict with each other. Sbc's exist only to fulfill embedded tasks where traditional motherboards will not work. (like in my car) I have typically found their performance to be "just enough" for what they were designed for and not much else.

CelGen wrote:

You need a dumb backplane that only supplies voltages to the slots and nothing or else as previously stated the boards will conflict with each other. Sbc's exist only to fulfill embedded tasks where traditional motherboards will not work. (like in my car) I have typically found their performance to be "just enough" for what they were designed for and not much else.

Then that would mean you could easily use an SBC without a backplane at all, and have it powering a small device as mentioned.

^^ Both my Advantech boards (PCA-6159 & PCA-6751) have provisions for running without a backplane (extra power connectors, etc.) The 6751 also has a PC/104 stack bus connector if you want to use it that way.

You only need the backplane if you want to use it in a more traditional motherboard-style installation.

Here's my PCA-6159-based DOS music workstation, in its 4U rack case and 14-slot backplane:

That was when I first got it, these days I've managed to populate at least 5 of the ISA slots and all four PCI. 😜

Edit: and for the curious, here's the 6751 - note the molex connector on the back of the SBC itself, that's how you supply power to it if you're not using the backplane.