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The adapter sold by TeamGloomy on Tindie for this purpose is sold out. This is my equivalent replacement.
I'm looking at putting ReprapFirmware on my Hypercube 300. The necessary adapter to connect its Raspberry Pi Zero 2 W to its SKR Pro 1.2 is out-of-stock on Tindie. This is a smaller adapter that does the same thing, only it plugs into the EXP2 header on the printer motherboard and a 6-wire cable connects the other end to pins 19-24 on the Raspberry Pi GPIO header. (I already have a 4-wire UART connection between the two that also supplies power; instead of putting in the full 40-pin GPIO header, it only has enough pins populated for the connections I'm using.)
It was a quick layout to throw together in KiCad; I probably spent more time playing around with panelization options than on the main layout. :) I should have 100 (20x5) bare boards arriving from JLCPCB, and the parts to go on it are dirt-cheap…should be well under $5 from DigiKey, and probably under $1 each in quantity from LCSC.
This project is also onprintables.com.