“I built a hacky USB interface to the Psion Organiser II that lets you send messages to it over USB serial via an Arduino Nano. It involves the organiser executing machine code stored in a string in its BASIC-like language, and it totally abuses the CPU’s bus design. But it’s simple and it works.
The motivation is that Martin wanted to use the Psion to display notifications, but couldn’t find anywhere to buy the “CommsLink” module that connects it to a PC. I thought it sounded like a fun Arduino project, so I did it.”