ROM emulators have become extremely common for classic consoles like the NES and Game Boy, but what about earlier computers that also used edge-connector-style expansions?
HP Series 80 group member Tim Nye developed a Teensy 3.6-based ROM-emulating plug-in module that allows users of the early 1980s pre-PC HP 83/85 and 86/87 to select any of the systems’ available ROMs to be loaded from flash memory.
The board includes some unique features, such as a 6V level shifter to bridge the HP’s bus with the Teensy’s 3.3V logic, and a diode to prevent the HP’s 5V power from flowing via USB to a connected computer during programming. One interesting challenge was that the Teensy needs to boot before the HP, but the
/HALT line on the HP’s bus can be driven by the Teensy to only permit the CPU to start once it’s ready.
The board appears to be a one-off with no information on purchasing or creating one’s own, but the HP Series 80 group is an active community, so drop them a note if you have a Series 80 with an empty expansion slot!