Bit Bang Serial
Josh has nice, interrupt driven, software serial routines in the Monitor of the Z80 Membership Card. They’re rock solid but, at 9600 baud, they’re a bit slow for loading large programs and they’re all mixed in with the code for refreshing the front panel display.
I’ve been looking for alternatives for a while and finally by looking for “software uart” instead of bitbang serial, I found what seem like good candidates. They’re written in Z80 assembly so they should be easy enough to incorporate in my bootloader. They’re not interrupt driven but that’s fine for my use case. I’ll only be calling them right after a reboot and I’ll just disable interrupts until I’m done. The author only says he has reliable results to 9600 baud so I may, in the end, not do any better but we’ll see.