The Psion Organiser II is a handheld 8-bit micro from the 1980s, it was sold to the public as a personal digital assistant (PDA) and was widely used in industry, over half a million were made. The PDA versions have a diary, can store information and can be programmed to perform calculations and manipulate data, there are even some simple games for it. Its PDA functions have since been replaced by the smartphone, so alot of these Psions eventually found their way into the back of a drawer somewhere. However, these devices are as tough as a brick, so if you find one and put a fresh 9 V battery in, it's very likely to work. The same cannot be said of many devices that followed it. The Psion II was a ground-breaking device in its time, and it still has an active following of dedicated users. Lots of info can be found on the site by Jaap Scherphuis:
I created this project as I wanted to transfer files to the Psion, and I don't have the Comms Link. The Comms Link is a serial adapter which plugs into the top of the Psion and provides a serial RS232 interface. These are becoming a bit rare and frequently sell for more than the cost of a second hand Psion.
There is a modern remake of the Comms Link with a USB interface, made by Olivier Gossuin, I recommend this for anyone who does not want to spend hours messing about with circuits and software. The Comms Link contains it's own software ROM which adds a lot of useful functions.
Another Hackaday project showed that an Arduino could be used to read/write to Datapaks, with the aim of using a Psion II as a bluetooth controller:
I started my project from scratch using info from Jaap and Olivier's web sites. Olivier's circuit diagrams of the Datapak and Rampak were very useful:
Psion's technical reference manual was essential:
I also took inspiration from this youtube video by Ben Eater: