System comprising mutually synchronizing first and second active functional units

A synchronization system for two active functional units which are interconnected by means of a synchronization connection. In order to obtain reliable synchronization without using an untoward number of wires in the connection, at least one of the two functional units has a generator which is capable of generating three successive discrete signal levels on a single connection wire of the synchronization connection. This function unit also has a detector for detecting a signal transition which is produced by a generator in the other active functional unit and for generating an activation signal in reaction thereto. In response to this activation signal, the generator of the same functional unit produces a succession of two signal transitions from one prevailing extreme signal level to the other extreme signal level of the three successive signal levels. The other functional unit has a detector which produces, in response to the second signal transition of the succession, an activation signal for the generator of the other functional unit in order to produce a synchronized signal transition or transitions. Such successions of signal transitions can be transmitted in one or both directions; the synchronization connection may even consist of a single connection wire on which a total of five different signal levels may then occur.