Data communication using constant total current

A data communication system, comprising:

• at least three signal conductors;
• a first and a second power supply terminal, for supplying currents of mutually opposite direction to the signal conductors respectively;
• a driver circuit coupled between the power supply terminals and the signal conductors, the driver circuit being arranged to establish a combination of currents through respective ones of the signal conductors, the driver circuit selecting successive combinations, depending on information to be transmitted, from a selectable set of combinations, at least three different of current levels to any signal conductor being used in the set, including a current level of current to the signal conductors from the first power supply terminal and a current level of current from the signal conductors to the second power supply terminal, a sum of the currents through the signal conductors substantially having a same value for each combination in the set and at least one of the conductors not merely functioning in a differential pair relation with another one of the conductors;
• wherein the driver circuit comprises an internal switchable current path for drawing current from the first power supply terminal to the second power supply terminal, the driver circuit activating the internal switchable current path depending on the combination being established, so that a first and a second net current, from the first and the second power supply terminal to the signal conductors plus the internal switchable current path respectively, each remains substantially the same upon switching between different combinations;
• and
• wherein the internal current path is arranged so that none of the current from the internal current path is capable of flowing to any one of the signal conductors.