It’s been an interesting morning. I’ve been digging around in the internals of the Siena SSD drive and made some discoveries.

The Siena SSD drive was released so that the diminutive Siena could still read SSDs. I bought one of these on eBay a week ago because I wanted to answer a question: “How did Psion get the SIBO Serial Protocol to work over RS-232?”

I now have everything I need to get started on revealing the secrets of the Psion SSD Port. So, here’s what I’ve been up to this morning.

Rather than pull apart my “production” 3c, I’ve decided to get to work on a slightly broken 3a. I don’t like the idea of doing something dumb thanks to my inexperience and blowing up my 3c. The 3a’s battery compartment is in a pretty bad way, but it powers on quite happily with a Series 3 external PSU. So, I pulled it apart and got to work.

Psion were a clever bunch, but like many companies in the early 90s they didn’t really do standards. Although the 3c and 3mx had a proper RS-232 serial port (albeit using a very odd connector), all of the earlier models used a proprietary protocol called the SIBO Serial Protocol. All Series 3 models used SSDs that also communicated using this proprietary protocol.

A significant part of developing this equipment involves working out how to emulate an SSD or Psion peripheral. Luckily, while trawling the Internet for Psion PDFs I found the Psion SIBO Hardware Development Kit. This book gives a breakdown of how to create equipment for the Psion Series 3 and 3a, including the controller chips needed, the Psion serial protocol, and how to write drivers for the Series 3 and 3a.