What happens to long lived electrical engineers? They diode!
I have had trouble with the Amiga 4000’s serial port since I got the machine. At first I got a few bytes through every now and then, but it was pretty much unusable, even at 9600 bps.
When “learning online” about all the magic of open source in GitHub, I stubbled about Keir Fraser’s Amiga Test Kit, John Hertell’s DiagROM and SukkoPera’s Parallel / Serial port tester for the two.
Of course it’s not worth just ordering a single one of anything in the electronics world, and so I ended up joining the market of dongle builders. If you need one, let me know.