I've worked out how to receive and transmit 3.3V TTL signals directly from/to the Psion Comms Link. This means the Psion can now communicate with 'modern' devices without needing complicated RS232-to-TTL converters and associated dodgy soldering...

Why 3.3V instead of 5V? It turns out that the HC-06 Bluetooth adapter, which I eventually hope to connect to the Comms Link to allow the Organiser & Pi to talk wirelessly, cannot cope with 5V signals on its TX/RX pins. The Comms Link emits 5V TTL signals, so I had to make a simple voltage divider using two resistors, to 'step-down' the 5V signal emitted by the Comms Link to the 3.3V required. It was then a matter of soldering a few wires on to the Comms Link PCB in the right places, found by (vaguely educated!) trial and error. It all seems to work nicely at 1200 baud. For now I've wired it up to the Pi's GPIO TTL pins. In short - The Pi-on is back in action!

The serial cable (which, avid readers will recall, terminates in a bulky DB25 connector) is now surplus to requirements, so I can cut it off to free up space inside the Comms Link's plastic case. Hopefully the Bluetooth adapter will fit inside the case, making the whole thing vaguely elegant...