freeVDV421

🚉 Open implementation of public transit IR positioning beacons according to VDV-421

What is this?

(At least some) German buses and trams use infrared communications to communicate with fixed roadside beacons. Basically, the conversation goes as such:

HELLO THIS IS BUS HELLO THIS IS BUS HELLO THI–

YEAH. GOTCHA. THIS IS BEACON 12345. THIS IS BEACON 12345. … YOU STILL THERE?

…

HELLO THIS IS BUS HELLO THIS IS BUS HELLO THIS IS BUS

So, the bus transceivers constantly announce their presence, while the roadside beacons mostly sleep (what a job!). Once a bus passes by a beacon, the beacon “wakes up” and announces its presence, transmitting whatever data they are pre-set to transmit. This could be their designation (re-setting the bus's odometry system, for location obtainment), or a code to submit to the next junction processor (think: traffic light preemption).

In turn, once the bus recognizes a beacon's transmission, it shuts up as not to interfere with reception. After a while, it re-starts shouting it's presence into the wild.

What is this project good for?

The associated standard and hardware date from the 1980s – yet, nothing in the way of a Free/Libre implementation exists. To get things straight: This is not about phreaking, haxx0ring or anything else script-kiddie-break-things-just-because-we-can-related. We do this, because we find it interesting and amusing, and because it opens the way for applications such as:

• tracking whether that bus is really always late, as you suspect
• Implementing a DIY-early-warning-system that reminds everybody at your space that the next bus just passed the corner two streets away
• Opening possibilities for next-generation public transit stops that gently shake the waiting bench as the bus arrives
• or simply because it is a fascinating field for testing your discrete electronics and/or microcontroller programming skills.

How does the IR transmission work, and how do you know that?

Through educated guesses, hints, and the practical application of reverse-biased photodiodes and a handy Digital Storage Oscilloscope we infer that

• buses modulate their data with a 28.8 kHz square carrier wave
• the data rate should be 4800 bps
• Encoding is Biphase Mark Encoding, with transitions on the 0

As you might already have figured out, that makes for just three pulses in a zero, and six pulses in a 1. Unfortunately, this rules out using standard IR demodulators like the TSOP3* models – since those need a series of at least 10 pulses, and pauses of at least 10 pulses in between.

Tools

/IR-Debugger contains an Arduino script for simulating a bus's IR transmission.