Bacnet Ms/Tp; Token Passing; Addressing - ABB Cylon FBXi Series User Manual

BACNET MS/TP

BACnet MS/TP (Master-Slave Token Passing) is an
devices such as Unitary Controllers. In comparison to BACnet/IP and BACnet/Ethernet, MS/TP is more cost-
effective to implement due to the lower cost of wiring. Given the MS/TP network is a serial-based network,
devices may be configured to communicate at different baud rates specified by BACnet. Therefore, it is
essential to know information regarding the BACnet network you are connecting to before installing.

TOKEN PASSING

BACnet MS/TP uses
controlled by each device, which contains an internal memory list of other MS/TP peers connected to the
network. The token is passed in order of the MAC Address (Unit ID) from lowest to highest. In most MS/TP
networks, each device is configured to be a master. Given all devices may be a master, MS/TP may appear
and react slower than traditional building automation protocols. However, configuring your network for
faster baud rates will help provide better bandwidth and transport speed of network messaging.
Token passing is a communications scheme that allows connected devices connected to intercommunicate
with one another. A network "token" is passed from unit to unit on the network in a round-robin fashion by
order of the MAC Address (lowest to highest) to provide a transport to access the network. When a unit
possesses the token, it may perform any network activity for which it is responsible. When finished, the token
is then passed onto the next device. At any time, the unit that possesses the token is the only device
permitted to initiate communications with another device on the network or to request information from it.
A device that receives the token may or may not need to perform network functions (e.g. read values from a
remote device, broadcast information, etc.). If not, it will simply pass the token along the network.
If you are connecting devices to an existing MS/TP network consisting of third-party devices, consult third-
party vendor documentation regarding MS/TP network considerations.

ADDRESSING

BACnet MS/TP devices contain two device addresses. One device address is known as a Device Instance, and
the other is a MAC Address. The Device Instance is an address assignment that is used to identify the BACnet
device on a global BACnet network. When a device is connected to a global BACnet network consisting of
multiple data layers joined together using routers, the Device Instance is used to uniquely identify the device
on a global basis. The valid range for the device instance in a BACnet device is 0 to 4,194,302. Devices must be
configured for a unique, non-conflicting Device Instance. In the event that multiple devices are assigned the
same Device Instance, both devices will simply not communicate on the BACnet network or could be subject
to misdirected messaging (a message intended for Device-A may be routed to Device-B)
The MAC Address is an address assignment used within the BACnet MS/TP segment to permit a device to
actively communicate on the BACnet MS/TP network. Valid MAC Address assignments range from 0 to 127
and are typically assigned in a logical and incremental order to permit faster token passing between devices.
The MAC Address of a BACnet MS/TP device must be a unique, non-conflicting value that exists on the local
MS/TP network. In the event that multiple devices are assigned with the same MAC Address, the effects can
be far detrimental than that of a conflicting Device Instance; potentially resulting in a failure of the entire
local MS/TP network. In the event that the unitary controller encounters a duplicate of its MAC Address,
devices will inform the user that a duplicate MAC Address has been detected and will not perform client
communications until resolved.
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token passing to allow devices to communicate on the network. Token passing is
BACnet® Networking
FBXi Series |
EIA-485 network layer intended for use with lower-level
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