Wow, thanks for the in-depth response!
Hopefully I did the idea justice first try.
Update RevB v.0.2:
Here’s the current schematic:
We got D0±, D1±, CN± and as promised the PCM interface on Mode B.
The camera connector has been dropped from the equation with that addition.
Some things:
- Minor mistake: I did cross MISO/MOSI on the communication lines, silly of me to confuse it with USART.
- Since I got the DevTerm today, I found out which connector to use for the fan, so that’s all components sorted.
- Will the MUX routing hold up?
I did use some vias and the lengths aren’t matched on the differential pairs (it’s not too that bad tough).
- I’m considering embedding python into the Teensy’s firmware.
You can’t flash it from the DevTerm, but wouldn’t it be neat to send some interpreter code over?
Plus you could embed the compiled interpreter code in the cart enumeration EEPROM. - Do we want to use a battery with the Teensy? Used to retain state when suspending and also provides an RTC. I think the SoC in the DevTerm already has one.
About those cart ideas…
a KVM thing
Neato, so basically a capture card with HID forwarded?
Too bad the USB device is already taken, tough theoretically possible if you port TinyUSB and bitbang it.
cartridge is a self-sustained system
Now that’s an idea, what if you put in an FPGA (plugging your thread here).
That is something that might get me into doing VHDL again. 
First thing tough I’m designing for it is just a plain RPI card. It’s hacker industry standard love it or hate it.
Also makes developing easier, talking to hats and such.
A GameBoy Camera type of thing would not only be cute, but interesting.