BACnet MSTP

BACnet MSTP is a token passing protocol. Here the MS stands for Master - Slave and the TP stands for Token Passing. This flavor of BACnet is mainly used for connecting field devices to controllers / routers / control applications. BACnet MSTP uses RS485/EIA485 as its physical layer which allows up to 128 devices to be installed on a single network with a max physical length of 4000ft and speeds up to 115k baud. Using repeaters allows the length to be increased. General baud rates are 19200, 38400 and 76800. All devices have got to operate at the same baud rate and upon autosensing the baud rate, more and more devices configure themselves correctly.

MSTP master and slave

Only nodes with the token are permitted to initiate service requests like requests for data. A device that receives a request, a request that requires a response, might respond without having the token. Based on this behavior it is simple to understand the difference between a MSTP master and slave. A slave is a device that can only send responses. A master is an initiator of a service request.