How would you do that ...

[grarpamp]

"Keyloggers" implied you only had broken boxes.
But now you say that you buy box.

There is no "serial USB to USB" cable.
There is "USB" cable.
There is "serial (TIA-232-F)" to "USB" convertors.

BadUSB exploits apply across USB to USB.
qr cd floppy tape sd as alternates.

You have more data options...
https://www.youtube.com/watch?v=kWp6hZ-5ndc

https://en.wikipedia.org/wiki/RS-232
https://en.wikipedia.org/wiki/USB
https://en.wikipedia.org/wiki/USB_flash_drive
https://github.com/topics/badusb

"firmware on your device is the NSA's best friend
it's reasonable to assume that all firmware is a cesspool of insecurity"


It's all workarounds,
throwing such oppressors out in revolt,
and not installing anyone in its place,
is the actual fix to the problem.
People trying to do that with computers has proven
to be a colossal inefficient waste of many years.
Go outside and play.