Transmission Voie-Machine - Background - Implementation - Electronics

Electronics

It is one of the more advanced railway signalling systems in the world, although the implementation itself is still based on relays rather than solid state electronics.

There are two components to the TVM-430 system: one ground-based, the other on board the train. Both run using Motorola 68020 class processors, and are programmed in Ada, a computer language often used in safety critical systems. The system makes extensive use of redundancy; the mean time between dangerous failures is estimated to be over 1 million years.

The ground-based segment of TVM-430 resides in trackside boxes, which control stretches of track about 15 km (9.3 mi) long. Each one is linked to the line's centralized traffic control centre, and directly controls about ten blocks of track, each with its own track circuit. Signaling information is encoded in AC signals which are fed into the rails of each block. There are four different carrier frequencies available in TVM-430 and they are used alternately in pairs on both tracks of the TGV line. On one track, blocks use alternately 1,700 Hz and 2,300 Hz, while on the other track blocks use alternately 2,000 Hz and 2,600 Hz. Upon these carrier frequencies can be modulated 27 separate audio frequencies, any combination of which can be present at one time; the earlier TVM-300 used eighteen separate frequencies, only one of which could be present at any time. Each block has a receiver at the opposite end to the transmitter, and the loss of the track circuit signal (owing to shorting by train wheels or due to a failure) is interpreted as an indication that the block is occupied. Signalling block boundaries are equipped with electrical separation joints that prevent adjacent blocks from interfering with each other, whilst still letting the traction return current (at 50 Hz) pass through. These tracks circuits are referred as a UM71 C track circuit.

The signals which are present in the rail are detected by antennas mounted underneath the front airdam of TGV trains, approximately 1 metre (3 ft) forward of the front axle. These antennas work by inductively coupling to the AC signal shunted between the rails by the first axle. There are four redundant antennas per train, two at each end. Only the two at the "front" of the train (in the direction of travel) are used. The signal from the track circuit is filtered, conditioned, and decoded on board the train by two redundant digital signal processors.