Maximum Number Of Bacnet Ms/Tp Devices On A Data Bus Segment And Baud Rate; Data Bus Segment Mac Address Range For Bacnet Ms/Tp Devices; Device Loading - Acuity Controls nLight ECLYPSE User Manual

BACnet MS/TP Communication Data Bus Fundamentals
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-
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vice addressing scheme (the MAC Address Range for BACnet MS/TP Devices). See
Range for BACnet MS/TP
Each device presents an electrical load on the BACnet MS/TP Data Bus Segment. This is called device loading. The
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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
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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
It is recommended that you connect no more than 50 ⅛ or ½-load devices on a single BACnet MS/TP Data Bus Seg-
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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-
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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-
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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
⅛-load devices
½-load devices
full load devices
Total Full-Load Devices
Table 5: 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
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Devices.
Baud
Quantity of devices
(example)
8
14
26
Using Repeaters to Extend the Data
Rate.
Equivalent full-
load devices
1
7
26
34
Bus.
Data Bus Segment MAC Address
Maximum devices supported by the manufac-
turer
128 Maximum 50 recommended
1
64 Maximum 50 recommended
32
There are too many devices on the data bus. It is
limited to a maximum of 32 devices.
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