BACnet MS/TP uses a token-passing protocol between the Master nodes of an RS-485 segment: the token holder is allowed to communicate during a bounded time slot and then passes the token to the next Master in the logical ring.

Token passing on RS-485 is the conceptual equivalent of a Token Ring. Only Masters can hold the token and initiate communications; Slaves only respond when polled by a Master and never hold the token. The rotating token defines the order of access, eliminates collisions and gives predictable latency. If a Master fails to respond when the token reaches it, the token is skipped after a timeout. In practice an MS/TP segment runs at 9600 to 115 200 baud and tolerates roughly 30 to 100 nodes per segment. The result is more deterministic than half-duplex Ethernet but slower.

When troubleshooting a slow MS/TP segment, measure the actual token rotation time first: most problems are diagnosed within the first minute by checking how long the token takes to complete a full loop.