Serial bus system

A multi-master serial bus system comprising a plurality of nodes, a communications highway over which transmission of data between the nodes can be effected, and access control means enabling only one node to access the highway at a time, wherein transmission of data from one node to another comprises an arbitration phase and a transmission phase, wherein the arbitration phase is divided into a plurality of slots, each node having an associated slot number, wherein there are n nodes and the arbitration phase is divided into 2n slots, which slots are divided into two groups of n slots, each node having a said associated slot number but which group it is in being determined by an arbitration scheme providing fairness of access of the nodes to the highway, wherein the highway comprises first, second and third lines, wherein the first line carries an arbitration clock signal during the arbitration phase, wherein the second line carries an arbitration signal during the arbitration phase and a data clock signal during the transmission phase, and the third line carries a slot signal during the arbitration phase and, when one said node has won access to the highway, carries the actual data to be transmitted from said node during the transmission phase.