This weekend I fixed the CAT interface of my radio (Kenwood TS-940), it was a couple of connectors with defective contacts (the white ones in the pic).
Then I connected to the PC via a USB to RS232 adapter and all was OK.
Then I tried to use my laptops and my Raspberry Pi 3: no luck with exactly the same Linux kernel.
So I turned on the oscilloscope to check the signals on the serial: as soon as I connected the probes the CAT worked. WTF?
The chinese adapter I was using didn't have the USB ground and the serial ground connected... How many hours did I lose? Fsck.