Okay, now I am back from the school run, might have a lead on those strange back to back PCI slots, after trawling around the microbus website (who made the backplane) on the way back machine, various other backplane/SBC makers website with not joy, decided just to search eBay for Microbus products and look at the pictures on that, and came across this odd SBC.

The Microbus MAT-818, with a parallel ISA/PCI design.

https://www.ebay.co.uk/itm/125615669809?hash= … ABk9SR7Sm__DWYg

Would fit for my board, but is this some sort of proprietary/solution by Microbus? Not paying £150 pounds for it though. Time to get the multi-meter out again and trace the connections...

Maybe this was a solution to add a pci (graphics) card to an existing isa SBC [and extend the pci bus onto the board] before it was common to put the pci slot behind the isa slot on SBCs.

The backplane shown seems to support both variants.

when i follow the traces on the board it seems the weird long double pci slot is for the unpopulated pci 64bit extensions on the normal pci slots.
Maybe there was a card to give a isa SBC PCI 64Bit abilities.

Well, with an PCI/ISA SBC in and configured to use PCI adapters, thought I would be able just to plug a PCI VGA card in (no VGA on my SBC board) and off I go, but no joy. The ISA slots all work so have VGA installed in that for the time, this is not going to be as easy as I thought, but then again when is it!

Voltages are good to the PCI slots, just not seeing a clock signal or other associated signals, so either missing a jumper somewhere on the SBC or this board needs something else?

Going to trouble shoot this further when I have got more time, have a P4 SBC but without a CPU so might try that later and my other backplane needs a bit of repair first before anything gets plugged into that.

these things are old PICMG standards for embeded/backplane stuff. search for picmg you will find loads of stuff. some backplanes were passive, some were active with pci controllers and stuff.

BloodyCactus wrote on 2023-09-21, 14:39:

these things are old PICMG standards for embeded/backplane stuff. search for picmg you will find loads of stuff. some backplanes were passive, some were active with pci controllers and stuff.

Searched through lots of information on that beforehand for anything on the back to back pci slots, but no joy, unless I missed something, which is possible, also checked the SBC before hand to see if it had a PCI controller on, which it does.

Got a feeling that it is a jumper setting or issue with the SBC, or an oddity with this backplane.

Got a few more to test and troubleshoot with.

DerBaum wrote on 2023-09-21, 10:16:

Maybe this was a solution to add a pci (graphics) card to an existing isa SBC [and extend the pci bus onto the board] before it […]
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Maybe this was a solution to add a pci (graphics) card to an existing isa SBC [and extend the pci bus onto the board] before it was common to put the pci slot behind the isa slot on SBCs.

The backplane shown seems to support both variants.

when i follow the traces on the board it seems the weird long double pci slot is for the unpopulated pci 64bit extensions on the normal pci slots.
Maybe there was a card to give a isa SBC PCI 64Bit abilities.

That is the direction I am thinking, can't find anything at the moment if this was a standard, so possibly a Microbus solution.

Quick update:

Been testing all the PCI connectors on the board, the PCI slot behind the ISA slot is only connected to the other PCI slots for power GND, 12V (-/+), +5V, etc.

The single PCI slots are correctly wired together, they connect to the reversed PCI above them, that PCI slot is correctly wired... but, for the card to be installed the other way around to the other cards!

So a conventional SBC with the ISA and PCI in line will not fully work in this board (ISA will work but not PCI), it must be designed for an SBC with ISA and PCI parallel to each other, similar to the SBC I saw on eBay possibly.

The 2nd backward PCI slot as previously identified only connects to the 64-bit PCI connectors (if they were there), rather interesting.

Another quick update. Been looking around and Ali Express seems to have a selection of SBC's which seem a fairly goodish price, so saved a bit of cash and been on a little shopping spree, on the way are:

another P4 SBC
Dual socket 370
I think a similar Microbus SBC to the one seen on eBay
a new standard backplane

Also a few 32-bit riser cables which will probably arrive before the above, to experiment with the Microbus back plane and see if my theory is correct.

Really excited about the dual socket 370 board though, comes with 440BX chipset and okay ATI 3D Rage XL graphics

My plan is to get a new testbench frame, mount a backplane along with a small factor power supply on that, which I can place in the spare space on my desk and build a plastic case around it, allowing me to test not just software but various cards saving the ware and tear on some of main computers.

Which means I can retire my thin client which is in the space, which has done its job well over the years, but it has emulated Sound Blaster FM sound through its VIA chipset, which really drags down the VIA CPU in it on some games.

Few bits have arrived, let the fun begin

The P4 SBC has 2.8 GHz chip and 1 Gig or RAM, thinking I will actually downgrade it a bit, find the slowest p4 I can and lower the RAM as much as possible.

The Pentium overdrive on the other card is mine, was running a few tests.

Prefer the P4 at the moment for this build wish it did not have built in VGA and audio like the other card, but the other card is playing up a little bit.

Not much going on at the movement, making slow progress as I await various parts to arrive.

Using the P4 SBC mostly, but not my favourite, it's PNP bus keeps taking over IRQ5 and 7 despite dedicating them to be ISA only in the BIOS, so the SB sound card is hit and miss at the moment.
P4 now is using a 1.4Ghz chip (all cache disabled) and swapping between 256MB/512MB of RAM, has it at the speed of a 486

The Socket 3 board, just will not work anything on the PCI bus despite all the jumpers being correct and having the ability to use PCI cards.

But I have a couple of other SBCs on the way still, so hopefully I will find one that works.

Going forward:
1) I am going with an PCI video card, either S3 or CL-GD type, using an S3 at the moment.
2) Like where the testbed frame is going, need to find a better way of mounting switches, i.e. power, reset, etc.. thinking some sort of big clunky industrial type switch
3) Sound, need to figure out what sound cards I am going to use, playing with a PCI one with General MIDI onboard Vs. SB Pro 2 + MIDI Card on the ISA side of life.

Been a fun Sunday, dual p3 board has arrived and now have a front panel of sorts.

Adding a sound card has been an absolute nightmare with these SBC's, despite what I do in the BIOS to assign the IRQ, it seems to make no difference. Only way I have found is to use the Intel Configuration Utility and finally got the SB16 working after a lot of head scratching.

The P3 board had one of the CPUs dead on arrival and RTC battery was dead (but to be expected), annoyingly the original battery was soldered to the board, so replaced it with a socket, but still the RTC clock and BIOS settings are getting lost when I power down, so must be another issue somewhere. Also no option for an external battery.

As before with the P4 board, going to downgrade to a slower P3 and lower the RAM, as my current thoughts are, now I have dual CPU's is to run NT 3.51, maybe even eComStation if I can find the disks, as I sort of remember that had SMP support. Of course, with dual boot to DOS.

So still more to do, such as tidy the wiring for the front panel, which is silly but I like it.

Wow, been a while, will post an proper update at some point. But:

Been using the dual P3 board for various experiments, but it is currently running NT4 Server for a few experiments, so plan to do the following:

1) Keep running NT4 but find a multi-port RAID card and install some small disk modules on it and create a little RAID 10 array to install NT onto
2) Tidy up the cables
3) Find some faster P3 chips
4) Upgrade the RAM to full capacity
5) Continue to tinker

As promised, nearly complete setup. Now running Windows Server 2000 as I had a copy in RAID 10 on a 3ware card (array of 4 x 8gb disks), installed the maximum 1gb of RAM and it is running the fastest P3's I could find which are 2 x 900mhz.

Now since the picture, I have added a TP-Link WiFi card (TL-WN851N v2), a 3DFX Voodoo 1 card (because I saw it all alone on my shelf and thought why not) and an AOpen sound card (AW744 type).