BAUD RATES
As a serial-based protocol, BACnet MS/TP supports the following four baud rates: 9.6kbps, 19.2kbps,
38.4kbps, and 76.8kbps. Devices can be configured for any of these baud rates, as well as native PC baud
rates 57.6kbps and 115.2kbps which are currently not supported by the BACnet standard. Each device
communicating on an MS/TP network must be configured for the same baud rate at all times.
NETWORK OPTIMIZATION
In BACnet MS/TP devices, specific device properties are available to permit optimization. Network
communications. By adjusting Device properties max-master and max-info-frames, users can adjust the
token passing abilities of devices. The functionality of these two properties is described as follows:
•
Max-Master
- defines the highest unit ID of an MSTP master that is connected to the network. This value
specifies to what address extent a token may pass. For example, if you have 64 devices addressed in a
logical order, this value would be assigned to 64. This value should be set to the same value across all
devices connected to an MSTP network.
•
Max-Info-Frames
- defines the number of data frames that an MSTP master can use the token before
passing onto the next device. This value is typically set by the factory but can be modified if necessary. In
the event a device does not need to keep the token for the number of frames specified, devices will
automatically pass the token onto the next device.
BACNET MS/TP DEVICE LOADING
MS/TP (Master-Slave Token Passing) is a protocol where each device is wired in series and they take turns
communicating, depending on which device currently holds a “token”. It is a robust design, and
simpler/cheaper than IP though less flexible in terms of interoperability.
BACnet MS/TP is widely used in building automation, and usually uses RS-485 networking. As a result, the
number of devices that can be connected together (on a “trunk” or “Fieldbus”) is limited by the electrical load
the device puts on the network.
Unit Load is a concept created by the RS-485 specification to help determine how many devices can be
connected to each fieldbus. The number of devices that can be connected depends on how much each device
loads the fieldbus so the more a device loads the fieldbus, the fewer additional devices can be used. The total
Unit Loads on a fieldbus must be 32 or less.
BACnet MS/TP allows 127 master device addresses, but the Unit Loading usually prevents that number of
devices being active on a fieldbus.
READ PROPERTY MULTIPLE
A single BACnet request can contain a sequence of BACnet property references, each representing a single
BACnet property. This allows multiple properties to be read with a single BACnet request.
By default, FBXi will read 5 properties at once.
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