32 Network Guide
BACnet MS/TP Data Bus Token-Passing Overview
The BACnet MS/TP data bus protocol is a peer-to-peer, multiple-master protocol that shares
data bus bandwidth by passing a token between Master devices on the data bus that autho-
rizes the device that is holding the token to initiate communications on the data bus. Once the
device has completed its request(s), it closes the communications channel, passes the token
to the next Master device (making it the current Master), and liberates the data bus.
The token is passed through a short message from device to device on the BACnet MS/TP
data bus in consecutive order starting from the lowest MAC address (MAC Address = 0) to the
next MAC Address.
Gaps or pockets of unassigned device MAC Addresses should be avoided as this reduces
data bus performance. Once a master has finished making its requests, it must poll for the
next master that may exist on the Data Bus. It is the timeout for each unassigned MAC
Address that slows down the data bus.
The way MAC Addresses are assigned is not a physical requirement: Devices can be daisy-
chained on the data bus in any physical order regardless of their MAC Address sequence. The
goal is to avoid gaps in the device MAC Address range.
Slave devices cannot accept the token, and therefore can never initiate communications. A
Slave can only communicate on the data bus to respond to a data request addressed to it from
a Master device. Gaps in slave device MAC Addressing have no impact on BACnet MS/TP
data bus performance.
1 Temporary commis-
sioning connection
Portable adaptor MAC Address for a tempo-
rary commissioning and maintenance connec-
tion
2 Reserved Other
3-127 Master Range Master devices: All Distech Controls' devices
are master devices and should be in this MAC
Address range
128-254 Slave Range Slave devices and network sensors
255 Broadcast Do not apply address 255 to any device
Table 7: Recommended BACnet MS/TP Bus MAC Address Values / Ranges for BACnet MS/TP Data
Bus Devices
MAC Address
Value / Range
Usage Devices
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