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 de-
vice addressing scheme (the MAC Address Range for BACnet MS/TP Devices). See Data Bus Segment MAC Address
Range for BACnet MS/TP Devices.
£ 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 de-
vice. See Device Loading.
£ 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.
£ It is recommended that you connect no more than 50 ⅛ or ½-load devices on a single BACnet MS/TP Data Bus Seg-
ment 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 controllers.
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 ini-
tiate communication).
£ Up to 128 devices (with device MAC addresses in the range of 128 to 255) that are BACnet MS/TP Slaves (cannot ini-
tiate 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.
Device Loading
Each device presents an electrical load on the BACnet MS/TP Data Bus Segment. This is called device loading. The use
of full load devices limits the number of devices connected to a BACnet MS/TP Data Bus Segment to 32 devices.
If a data bus segment is interoperating with devices that are full-load, ½-load, ¼-load, or ⅛-load, then the device that sup-
ports the fewest devices on the same data bus is the one that sets the limit for the maximum total number of devices for
that data bus segment. For example, you plan to put on one data bus the following devices:
Manufacturer Quantity of devices
(example)
Equivalent full-
load devices
Maximum devices supported by the manufac-
turer
⅛-load devices 8 1 128
1
Maximum 50 recommended
½-load devices 14 7 64 Maximum 50 recommended
full load devices 26 26 32
Total Full-Load Devices 34
There are too many devices on the data bus. It is
limited to a maximum of 32 devices.
Table5: Device Loading Example
1. This is limited by the maximum number of master devices allowed on a BACnet MS/TP Data Bus.
The solution for the above example is to create two data bus segments connected together by a repeater and then split up
the devices between the data bus segments, ensuring again that the maximum number of devices on each separate data
bus is not exceeded. See Using Repeaters to Extend the Data Bus.
BACnet MS/TP Communication Data Bus Fundamentals
92
nLight ECLYPSE
To Next Page
Loading...
Need help?
General