Before smartphones came along, the pinnacle of ultra-portable computing was the PDA (Personal Digital Assistant). Several manufacturers produced models over the years and the most famous is the PalmPilot line, but they all served the same purpose: to put computing power in the user’s pocket. Some very late models had the ability to connect to the internet via WiFi or wired dongles, but most models didn’t possess that connectivity. To get his old Psion PDA on the internet, Kian Ryan built a Raspberry Pi-based “sidecar” RS232 terminal. Low Cost Handhelds
Ryan’s device is a Psion 5MX, which is a PDA launched in 1999. It has a full QWERTY keyboard, respectable 5.6” grayscale LCD with a 640x240 resolution, a 36MHz 32-bit Arm processor, 4-16MB of RAM, and can run for up to 20 hours on a pair of AA batteries. But it can’t access the internet or networks at all — at least not in the way modern devices (and even computers of the era) do. The Psion 5MX does, however, contain some hardware for communication: an RS-232 serial port and infrared. The latter is extremely slow and is really only suitable for sharing tiny amounts of data between devices in close proximity, but the RS-232 port is the key to this project.
Serial communication is still in use today and so many modern devices support the protocol. In this case, the Psion 5MX communicates with a Raspberry Pi Zero W via RS-232. To make that easy, Ryan designed his own RS-232 and null modem HAT for the Pi Zero. To make this ‘sidecar’ self-contained, Ryan added a Pimoroni LiPo SHIM, Pimoroni LiPo Amigo Pro, and a 2200mAh LiPo battery. He crammed all of those into a custom 3D-printed enclosure to keep things tidy.
The Raspberry Pi can run in headless mode and the Psion 5MX can access its terminal through the RS-232 connection. There, it can do anything in the terminal that a user would be able to do if they had a keyboard connected to the Raspberry Pi itself. But Ryan took this a step further and set up the two for PPP (Point-to-Point Protocol). That lets the Psion 5MX use the Raspberry Pi’s internet connection through the RS-232 connection. Any packets the Psion sends will go through RS-232 to the Raspberry Pi and then out to the wider internet, and received packets follow the same path in reverse.
Of course, the Psion 5MX has very little software that can take advantage of the internet. But there is some! Ryan was able to install the Opera web browser on his Psion and used that to access websites — though most are secure these days and aren’t accessible to old hardware. Telnet, on the other hand, still works well.
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