14 Network Guide
Maximum Number of BACnet MS/TP Devices on a
Data Bus Segment and Baud Rate
The following technical parameters limit the number of devices on a BACnet MS/TP Data Bus
Segment.
• The BACnet MS/TP Data Bus Segment has a hard limit on the number of devices that can
communicate due to the device addressing scheme (the MAC Address Range for BACnet
MS/TP Devices). See Data Bus Segment MAC Address Range for BACnet MS/TP
Devices on page 14.
• Each device presents an electrical load on the BACnet MS/TP Data Bus Segment. This is
called device loading. The number of devices that can be connected to a BACnet MS/TP
Data Bus Segment is limited by the loading of each device. See Device Loading on page
15.
• Choosing a low baud rate can cause BACnet MS/TP Data Bus congestion that can limit
the amount of data that can be efficiently exchanged between devices connected to the
BACnet MS/TP Data Bus. For example, at 9600 baud, the maximum number of devices is
reduced to 25 due to the increased time it takes for token passing between devices. The
recommended baud rate is 38 400. See Baud Rate on page 16.
• Distech Controls recommends that you connect no more than 50 of our ⅛ or ½-load
devices on a single BACnet MS/TP Data Bus Segment when a baud rate of 19 200 or
higher is used (preferably 38 400 baud). This is to ensure that the BACnet MS/TP Data
Bus has enough bandwidth to efficiently communicate network variables between control-
lers.
These parameters are described in greater detail below.
Data Bus Segment MAC Address Range for BACnet MS/TP
Devices
The BACnet MS/TP data bus supports up 255 devices:
• Up to 128 devices (with device MAC addresses in the range of 0 to 127) that are BACnet
MS/TP Masters (that can initiate communication).
• Up to 128 devices (with device MAC addresses in the range of 128 to 255) that are BAC-
net MS/TP Slaves (cannot initiate communication).
However, it is recommended that any given data bus segment have no more than 50 devices,
when a baud rate of 19 200 or higher is used for the BACnet MS/TP Data Bus. A repeater
counts as a device on each data bus segment to which it is connected.
All Distech Controls' devices are categorized as BACnet MS/TP Masters, that is, their device
MAC address can be set in the range of 0 to 127 only.
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