Data communication system

In a ring network (1) messages sent from any one of a group of stations (3) to all the remainder include an acknowledgement field (ACK) holding a count preset to equal the number of expected destination stations. Each destination station that successfully receives the message acknowledges by decreasing the count by 1; acknowledgement is withheld if an error is detected in the message or the station cannot accept it. Acknowledgement is performed by a circuit (17) in a loop-through path (15) in an interface unit (4) connecting the station to the network (1). The originating station checks in a circuit (28) if the final value of the count on its return is zero. If so, the originating station is assured that each expected destination has made a positive acknowledgement of successful receipt.