Abstract: In a communication system which ensures that a slave device can successively transmit a plurality of answer messages without causing collision with another message on a communication line, in response to a request message from a diagnosis tester, control devices transmit their answer messages in the order of their priorities, with a communication blank duration provided between answer messages. If any one of the control devices has first and second answer message to be transmitted successively, the control device transmits the second answer message a communication blank duration after completing transmission of the first answer message. The control device can thereby transmit the first and second answer messages before a control device of a lower priority starts to transmit its answer message. Message collision on the communication line is thus avoided.

Abstract: A master device sends a continuation (or stop) message on a communication line without causing mutual collision on the communication line with a response message continuously sent from a slave device. When time T1 has elapsed since completion of sending of a request message, a failure-diagnosis tester 5 (master) performs setting to send a continuation message when an idle counter has exceeded P3', and along with this, an engine controller 1 (slave) performs setting to send a subsequent response message when an idle counter has exceeded P2' (>P3'). Through this, the continuation message can be sent from the failure-diagnosis tester 5 prior to the engine controller 1 sending the subsequent response message, without causing the two messages to mutually collide on the communication line.