I'll say it again:
Replace The Backplane with Wires
As in 1 (one) SPI channel PER HASHBOARD going direct to a controller. I hope to god you didn't have all the hashboards sharing a common bus on the backplane before - because I can't imagine trying to get that to work.
Dear Jonny,
The PCIE connector has 98 pins and there are 5 connectors in each case.
If you still believe that this can be done even on one X3 (not to talk about mass production) you are welcome to come to our office and try to do this yourself.
Regards!
The PICE connector has 98 pins but many of them are bussed together for power. In the photos on your website I count 18 data pins on one side of the card and the rest - bussed together for power. An 18 conductor ribbon cable isn't that hard to come by, and then it would just be a matter of getting enough power to the board. I could be wrong - and I can't say I still believe this is possible because I don't have a full understanding of the PCB layout - only what I've seen in scattered photos.
A few questions before I "book my ticket":
1) What is the pinout of the hashboard?
2) Does the board create 1 SPI link between the raspberry pi when it is working, or 2 (one for each chip)?
3) Does the backplane now bus together all the SPI links from the hashboards, or is there a single isolated link between each hashboard and the controller?
4) Does the SPI "Daisy Chain" - that is to say go from the first hashboard to the backplane and into the second hashboard then back out to the backplane so on and so forth. If so has the ability of the hashboard to support this daisy chain link been verified?
I'm honestly trying to help, not trolling, and I'm a little dismayed by your dismissive response to my sincere suggestion - especially considering you guys openly asked for help.
Oh, one more thing. Even if it was all 98 conductors active there are commercially available parts for that.
http://www.frozencpu.com/products/20753/cab-1908/PCI-Express_PCI-E_x16_Extension_Cable_Riser_-_7.html