Product manual

B Protocols TAC Xenta Server – Gateway, Technical Manual
152 (184) Schneider Electric Buildings AB, June 2011
04-00124-06-en
B.5 BACnet MS/TP (Master Slave/Token Passing)
The Xenta 913 can be configured to connect to a BACnet MS/TP serial
network to allow monitoring and control of one or more devices through
an I/NET or LON control system.
A number of BACnet Objects can be connected to a corresponding set
of LON Network Variables or I/Net Points to allow one or more Master
or Slave devices to be monitored and controlled. The Xenta 913 acts a
master, but is designed to coexist with other masters on the MS/TP net-
work if present.
B.5.1 BACnet MS/TP Networks
Each BACnet MS/TP Network consists of a number of independent
masters and slaves interconnected by an RS-485 serial link. A master
can poll slaves to read their data values, or can write data values to the
slaves if applicable. Some masters may also operate as a slave on the
network.
The Xenta 913 operates as a master only. Once it has successfully
joined the MS/TP Network it continuously polls the attached devices to
read the required data values for use within a control system. It can also
write the necessary control system values out to the slaves. Other mas-
ters can operate independently on the network, although each master is
required to share access using token passing.
Each master and slave device must have a unique numeric address on
the network. Slave addresses can be in the range 0 to 254, but master
addresses must be in the range 0 to 127. Furthermore, to minimize the
time taken to establish network communications, master addresses
should be restricted to less than 127 wherever practical.
A maximum of 32 master and/or slave devices can be physically con-
nected to an MS/TP Network. If more devices are required one or more
RS-485 Network extenders can be fitted.
Fig. B.5: BACnet MS/TP
Value exchange
BACnet MS/TP Network
Slave Slave Slave
RS-485 A
Master
BACnet MS/TP
LON or I/NET Control System
Xenta 913