The digital tachograph is composed of a Vehicle Unit (VU) essentially containing a printer, two slots for the cards and a display as well as a sensor connected to the gearbox. The VU is the brain of the system. It is able to hold data on drivers of the vehicle and their periods of driving and duty for about a 12 month period. It also and notably holds data relating to faults, attempts to tamper with the system, over speeding, calibration details, and when data has been accessed, for example, by the Police.

The VU and the motion sensor from the gearbox are paired and the signals from the sensor are fully encrypted so that any attempt to interfere with them is registered and recorded in the vehicle unit. The VU is set to Universal Time Co-ordinated (UTC) – as another name for Greenwich Mean Time (GMT) - all records are against this time. The visual display can be set to the local time, but this does not affect the internal time.

It functions with smart cards. Drivers, companies (operators), workshops (tachograph calibration centres) and enforcement officers (Police for example) are issued with smart cards according to their specific needs. These enable them to use and / or give access to the data in the VU.

Whenever the words “digital tachograph” are used, it has to be understood as recording equipment.