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This is dumb, but doesnt it seem like the 8,1 kind of has 8 slots. There seem to be 2 apple IO cards up top with 6 free.
Why are they counted those 2 cards as just one slot?
On the MP 2019 Apple used discrete Intel Thunderbolt controllers to provision the four TB sockets. On slot 8 there was a TB controller and a USB Controller (or at least USB PHYS from a USB feed from the Intel PCH).
Our OWC friends Tom and Brady have torn apart a 2019 Mac Pro. Not only do we have it on video, but check out our high-res photo gallery of all the parts as well!
eshop.macsales.com
old Slot 8 was a 'mini MPX' augment to the standard slot. Slot 8 had some proprietary 'augment' pins from main board that carried GPIO and two DP out to the 'old' Apple I/O card to provision the inputs to the TB controller there. I think the USB is being run out through that augment connector also from the PCH ( and only USB PHYS and Power Support chips out there on the card). [ there is audio on that card too. ]
The card on slot 8 now has
NO CONTROLLERs. Thunderbolt is provisioned entirely out of the SoC. It is already a TB signal when it gets to the card. There is some USB PHYS and power support chips to support USB -C sockets requirements and likely a redriver to remove any signal loss from crossing the large main board, but there is zero need to run PCI-e lanes out there at all. It is gone. It pragmatically entirely becomes an entirely proprietary slot. The only thing on that card is TB and TB doesn't need PCI-e at that point in the slightest. (there is PCI-e controller built into the TB controller on the main die. )
Very similar reason why the MPX sockets are gone. Thunderbolt comes out of the SoC and not done "at the edge' of the system.
New slot 7 has USB and HDMI ports on it. Likely Apple is reusing that 'augment' pins to run DP video out there where it is converted into HDMI at the edge. They may be running PCI-e to a discrete USB controller . ( Step 10 of iMac 24" teardown
https://www.ifixit.com/Teardown/iMac+M1+24-Inch+Teardown/142850
The 'extra' USB-C sockets are run by a ASMedia PCI-USB controller. Same theme of putting ports on a seperate I/O card that can replace if someone manages to completely blow out the circuits behind the port. ). There would be PCI-e out to that controller.
So new Slot 7 is augmented like 'old' slot 8 was. So same option there to yank the card and loose 'standard' Apple provided I/O ( only HDMI and USB and Audio this time ). and put your own in.
I suspect there will be folks who want optical audio out instead of a headphone jack and are happy enough with type-C DP to a general market monitor and don't need USB sockets. Or need yet another Ethernet Jack instead
View videos and tech specs for McFiver PCIe card, and determine hardware and OS compatibility.
www.sonnettech.com
and just do digital out through the TB ports. [ The 'fiver' packing multiple things onto a single card because 'down one' slot makes sense. ]