I still find the #HP9816 useful today, to control #GPIB instruments. I wrote two programs, one to control the Bruel&Kjaer 2034 audio spectrum analyzer to do third-of-octave analysis and another one to do various measurements with a #HP4277A LCZ meter. If I need to further process data, I download them on a modern computer using a RS232 cable I built. I transferred and published the HPBASIC source of the third of octave software on Github: https://github.com/DarwinNE/BK2034_GPIB/tree/master/HPBASIC

#GPIB (what ultimately became IEEE-488) originated with HP as an instrument control interface in the 1960s, but Commodore adopted it for its earliest peripherals (of course, this being Commodore, not using the standard connector, but a cheaper edge connector).

It was used in the PET era, but the very first disk drive I had for my #C64 was actually using an IEEE-488 cartridge (might have been a 2031 drive, but I have no record of it).

Even though with its parallel nature, it was way faster with regular kernal I/O than Commodore's later serialised version of the interface, I didn't like it at all at the time because so many games and demos blindly assumed to find a 1541 connected and did not work with it.

Sure wish I'd held on to that now... ​