The Vehicle Unit (VU) is located within the driver’s area of the vehicle cab. It sends a signal to the speedometer/odometer unit that is located where the driver has a clear view of it. The Vehicle Unit still receives a signal from the vehicle (usually from the gearbox) as the analogue units do, via a cable.
The VU is the brains 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 holds data relating to faults, attempts to tamper with the system, over speeding, calibration details, and when data has been accessed, for example, by VOSA staff or Police.
The VU and the motion sensor from the gearbox are encoded as a pair and the signals from the sensor are fully encrypted so 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.
Drivers, companies (operators), workshops (tachograph calibration centres) and enforcement officers (VOSA & Police) each have smart cards according to their specific needs. These enable them to use and / or give access to the data in the VU.
There are three main Tachograph Manufacturers; Stoneridge Digital Tachograph, VDO (Siemens or Continental) Digital Tachograph and Actia Digital Tachograph.