Riding in Sidecar: How to get a Psion online in 2023

Using a gigahertz-class computer to get an 36MHz computer onto the internet – or even just printing

Code whizz and tinkerer Kian Ryan's ingenious "Sidecar" is a self-contained, battery-powered Wi-Fi-to-RS232 bridge that enables his elderly Psion 5MX PDA to access a little bit of the modern internet.

The Sidecar is an inspired homemade device which combines an assortment of bits of hardware and software, a 3D-printed case, plus considerable ingenuity, to create a pocket-sized gadget that turns a vintage mobile computer into a mobile Linux terminal – and which can also permit elderly gadgets access the internet.

You can trace the development of the Sidecar over six months on Ryan's blog. Back in June, he described a fairly complicated setup. This used a MAX3232-based [PDF] RS232-to-TTL level converter, plus a gender-converter and a null-modem adapter to hook a Psion serial cable to a Raspberry Pi Zero. (He didn't mention it, but we think it must a Pi Zero W to provide an upstream network connection.)

That worked, but he suffered from buffer overflows with large transfers so he added a few more wires, plus rpirtsrtc to enable hardware flow control, and AtariSIO to manage it to get hardware RTS/CTS flow control working. It's clever, but it gives The Reg FOSS desk distressing flashbacks to 1980s serial-port troubleshooting sessions.

A subsequent post in November simplified this setup by building an SP3232 [PDF] transceiver and the other components into a single Pi Zero "MicroHAT" based on an existing template. This is even more impressive, but it was not a smooth process. He's published the design on GitHub, which uses KiCAD for the layout. He told The Reg:

The sidecar isn't a difficult build: all through-hole components, and the print is straightforward. The 232 chip is no longer in production as DIP package, but is easy enough to get hold of on eBay.

The final step was building it into a case, along with some Pimoroni bits: a battery pack, plus a LiPo-Shim and LiPo-Amigo to power the Pi Zero and charge the battery respectively. He designed his own case, for which you can download the pattern from Printables, and printed it on his Prusa Mini.

His blog post describes how to set up EPOC32 to make a PPP connection via the Sidecar, and how to set up a web browser, email over both POP3 and IMAP, and how to find old HTTP-only websites that are still accessible from a nearly quarter-century-old computer.

Even so, the Sidecar is not dedicated to the Psion 5MX by any means. Ryan has also described how to use the Sidecar to get online with a modern CP/M computer, the RC2014. We suspect that you could also do this with other retro tech such as a Cambridge Z88 or TRS-80 Model 100, for example, with ZSock. It should also work with a Psion Series 3A. It's still possible to download the EPOC32 version of the Opera web browser, or Psion's own PsiMail for the 3A.

It's an ingenious setup, and we particularly admire the way he's made it so compact and able to run off a battery or USB power.

We suspect that it wouldn't be a huge amount of work to exchange the RS232 port for Ethernet, such adapters exist. That could turn the Sidecar into a Wi-Fi-to-Ethernet adapter. Several guides can walk you through setting up the Pi OS to do this, with or without using a GUI.

Babbage Difference Engine No. 1

A history of personal computing in 20 objects part 1


Another ingenious bit of kit that uses a Raspberry Pi to allow vintage computers to connect to modern devices is the RetroPrinter, from Sinclair spare-parts suppliers RWAP Software. This uses a Pi HAT to give the Pi a Centronics parallel port. Software on the Pi takes the data from an old computer, or other device, that can generate ESC/P2 or HP PCL output, and rasterizes it for output on a modern USB-only printer. ®


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