Abstract: A data communication system comprises a main device with a first microcomputer and an operating device with a second microcomputer. The devices are connected to each other by a first wire and by a second wire. In addition to that, input interrupt terminals of both devices are connected to the second wire. One device starts the sending of data by pulling the second wire to a first level, the detection of this by the other device is signalled back via the first wire and both devices then start an internal timer. When a predetermined time, corresponding to a data item to be sent, has lapsed the sending device pulls the second line again to the first level. This is detected by the other device, which measures the lapsed time and which uses this time to determine the data item sent by consulting preset conversion rules, that give a relation between time and a data element.