Hendrik Boom <hendrik@???> wrote:

>> That's PS/2, not RS232.
>
> True, true. And there seem to be two different sizes of those
> round plugs in use. At least, I've seen adapters to connect between
> the two sizes.

I think you may be thinking of the original PC keyboard connector which was a standard DIN plug most commonly used in audio applications (and MIDI). https://en.wikipedia.org/wiki/DIN_connector
The PS/2 keyboard & mouse connectors are 6 pin mini DIN (aka MCC, Mini Circular Connector) connectors, also made popular by Apple's use of the 8 pin version for the serial ports on the Mac. "Somewhat fiddly" to solder wires into, especially if the wires are a bit on the thick side ! https://en.wikipedia.org/wiki/Mini-DIN_connector

AFAIK the underlying protocol for the keyboard is the same (or near enough for simple conversion) between the two connector formats to allow for easy conversion between plugs.