One of the things that was good about retro computers was the User Manuals. In my Z88 manual, it describes how to make an RS232 cable that will work with its serial port (the port on the Z88 has a non-standard pinout). So I made one:
It was made from bits that I had lying around (as you can tell). Mostly stuff salvaged from old PCs in fact. Note the USB adaptor on the end. It's clunky ... but it works. I could easily make a better one (one not made from junk perhaps).