Abstract: A network provides communication between end units (1). Each end unit (1) received an entitlement to transmit in the form of a go-ahead. It is then able to transmit a frame of user data, which is broadcast by a central unit (7) to all other end units (1). When the end unit (1) transmitting the frame wishes to relinquish its entitlement to transmit it regenerats the go-ahead, which is passed by the central unit (7) to the next end unit (1) taking them in a predtermined cyclic sequence. The system allows frames to be acknowledged: the central unit (7) collects acknowledgements from end units (1) receiving the frame successfully, and when every expected station has acknowledged, returns an acknowledgement to the source end unit (1). Central units (1) may be combined to form larger systems.

Abstract: 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.