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, CBXi will read 5 properties at once.
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