The BACnet protocol MS/TP communications bus is a local network that connects supervisory controllers and equipment controllers to field point interfaces. The bus is based on BACnet standard protocol SSPC-135, Clause 9.
The BACnet MS/TP protocol is a peer-to-peer, multiple master protocol based on token passing. Only master devices can receive the token, and only the device holding the token is allowed to originate a message on the bus. The token is passed from master device to master device using a small message. The token is passed in consecutive order starting with the lowest address. Subordinate devices on the bus only communicate on the bus when responding to a data request from a master device.
An MS/TP bus is used for two types of buses: a Field Controller bus (FC) and a Sensor Actuator (SA) bus (Figure 1).
The FC bus and SA bus are networks of daisy-chained devices. Each bus has only one bus supervisor, depending on which controllers are connected. On a local FC bus, the bus supervisor is the BACnet router. On the local SA bus, the bus supervisor is a field controller.
The bus supervisor communicates with devices on the supervised bus and with devices on the next (higher level) bus on the network. The bus supervisor typically starts the communication on the FC bus or SA bus. If an SA bus or FC bus does not have a bus supervisor, the master device with the lowest device address value on the bus and a specific baud rate selected starts the communication.