Hiya, I was working on this PM before I saw you post:
You'll probably find the CSII pinout it in any of the CSII equipped series Developer Notes. I was looking at the 5400 DevNote earlier and found it. Apple calls it the
PCI-Bus Communications Slot. Maybe someone has a link to that handy or to any other CSII machine DevNotes? If not, PM an addy and I can email it to you, haven't got the original title of the PDF for searching.
Looked after you posted the thread, but I didn't see CSI in theQuadra/LC 630 series DevNote though.
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In a tangent in another thread, you'd mentioned wondering about RPI Zero W for CSII Networking and a general lack of SBC compatibility with PCI, which sent me right off on a search:
Article about the hack free RPi CM4 version with PCIe:
https://www.electronicdesign.com/technologies/embedded-revolution/article/21145132/electronic-design-its-here-the-raspberry-pi-cm4-with-no-edge-connector
Article about hacking off the USB3 controller from a standard Raspberry Pi 4 to get at its PCIe interface:
http://labs.domipheus.com/blog/raspberry-pi-4-pci-express-it-actually-works-usb-sata-gpu/
YouBoobTube fun look at that hack:
https://datasheets.raspberrypi.org/cm4/cm4-product-brief.pdf
The full-on carrier board for CM4 appears to have a slot supporting its PCIe Gen 2 x1 interface. I'll make a working assumption it can be made to communicate bidirectionally with the "PCI-Bus Communications Slot Connector." The reduced 55mm × 40mm board size of the new CM card and interconnect fits it well enough into CSII Slot card cubic and onto some magical carrier card for that slot?
That's as far as I got. It's all Greek to me, but fun finding the stuff. [

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edit: I see you've found the CS(I) pinout. moar:
https://www.raspberrypi.org/blog/raspberry-i-compute-module-4/
I think I got all that into my reply in the new post, I hope it's helpful. Fab idea on slotting RPi into the PCI-Bus Communications Slot, hopefully it won't need to be jusdt a serial connection! [

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jt