Topology
Much Bisync traffic was point-to-point. Point-to-point lines could optionally use contention to determine the master station. In this case one device would transmit ENQ to bid for control. The other device could reply ACK0 to accept the bid and prepare to receive, or NAK or WABT to refuse. In some cases connection of a terminal to multiple hosts was possible via the dial telephone network.
Multi-drop was part of the initial Bisync protocol. A master station, normally a computer, would sequentially poll terminals which are attached via analog bridges to the same communication line. This was accomplished by sending a message consisting only of an ENQ character addressed to each device in turn. The selected station would then transmit a message to the master or reply with EOT to indicate that it had no data to transmit.
Read more about this topic: Binary Synchronous Communications