Everybody knows them, hardly anybody thinks about them: the large LED displays at bus stops and stations that tell you in little orange dots which bus leaves from which bay and when. In German public transport jargon they are called DFI displays, short for dynamic passenger information. And many of them share a problem you can't see from the outside: the display itself is perfectly fine, but the computer behind it is hopelessly outdated.

Exactly such a display was hanging at the train station here in Ravensburg. Together with my team at Lulububu, first and foremost my colleague Michael Malura, we opened it up, looked at what is actually inside and gave it new internals. The result is live by now, and the most exciting takeaway is this: it doesn't only work for this one display.

The first time we opened the housing, it was quickly clear where the problem was. The display itself is a solid standard unit made by deister electronic, not a custom build: a double-sided LED matrix with 240 × 120 pixels, driven by 16 serial controllers, enough room for twelve rows with line, destination, departure time and bay. The hardware passengers actually see is built to last and would still have many years ahead of it.

The real troublemaker sits behind it. The whole thing was controlled by a deister VDX-6326 board computer that still runs Windows XP Embedded. Timetable data came in through an old proprietary protocol called IproM via GPRS, meaning over the 2G mobile network, whose days in Germany are numbered. An operating system that hasn't seen updates in years, attached to a data connection that soon nobody will offer anymore: that is not a foundation you want to keep running a display on.

The display connects to the internet through a 4G modem (Waveshare SIM7600) with its own SIM card, secured by a WireGuard VPN. Instead of the old IproM protocol, the Raspberry Pi fetches the departures directly from the transport association's API. It shows the buses of the next two hours, and it does so with real-time delays, not just the scheduled times from the timetable.

Since our own software now runs on the display, it is no longer just a dumb screen but a device you can control and monitor remotely. An admin backend lets you push texts such as service disruptions or notices to every single display, for example when a line is diverted because of roadworks.

Looking the other way is just as important: for every device, the backend shows live whether it is online or offline, how good the LTE signal is, how much data it uses, how warm it is inside the housing and how long it has been running. If a display fails, you don't find out when passengers start complaining, you know right away.

And since we are software developers after all, we obviously had to find out what else such a huge LED matrix can do. The answer: Snake. Controlled with a phone, played on the big display at the station, before it had to go back to politely showing departure times.

The first display is running, and that is where it actually gets interesting. Because deister displays of this kind don't only hang in Ravensburg. Wherever the same LED matrix with the same serial controller bus is installed and the display itself is still intact, our retrofit can basically be transferred one to one: board computer out, Raspberry Pi, adapter, modem and cables in, done. That is a manageable, predictable effort per location instead of buying a completely new system.

To be honest, it also has to be said that not every display is a sure thing. If a unit comes from a different manufacturer or the display itself is broken, that is a separate case we need to look at individually. And if you prefer open industry standards over a direct API connection, VDV 453/454 is an obvious alternative to the old IproM protocol.

Are you a transport company, a transport association or a municipality with departure displays whose technology has aged? Maybe they also still run an old Windows computer, maybe their data connection depends on the fading 2G network, or maybe the display doesn't show anything at all anymore. Then it is worth taking a look behind the cover before you think about an expensive complete replacement.

Just write me a message, ideally with a few photos of the display and, if available, its type plate. Then we can quickly assess whether our retrofit fits your displays as well and give them a second life.