RasmusB on Hackaday.io is resurrecting a Psion Series 5 PDA:
Bringing a Psion Series 5 into this decade by replacing all the important bits.
The completed result will (hopefully) be a portable modern Linux system with all the connectivity options expected in a modern device.
The keyboard adapter board is available on Tindie:
This is an USB interface for Psion series 5 PDA keyboards. Plug in a keyboard and a USB cable, and use it with any modern computer!
The design files and source code are available on GitHub: