User Manual

Overview
Networking BACnet network
3
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For detailed information, see
Desigo Ethernet, TCP/IP, MS/TP and BACnet (CM110666) and Practical
guide on IP networks in building automation and control
(CM110668).
3.3 Networking BACnet network
The topics of IP networks and BACnet merge when networking BACnet networks. Standard knowledge of
planning and implementation suffice for small to mid-sized projects. Expert knowledge of BACnet (and not
necessarily IP networks) is required to plan, size, and implement large and complex BACnet/Desigo
projects.
Desigo is based on that standard BACnet communication protocol. Services are required to communicate
between BACnet devices. The following can be used to exchange data, for example, to transmit a demand
signal from the room and its evaluation and forwarding to the primary plant:
● A Unicast service continuously queries and evaluates the state using a cyclical read (ReadProperty).
● A Unicast service that subscribes to the demand signal (SubscribeCOV) for a specified lifetime and
receives an update if the value or its state changes.
This exchange of data between both BACnet devices can be labeled as highly targeted. The communication
protocol also supports broadcasts to all BACnet devices. Example:
● A BACnet client determines at start up all pending alarms and events to display them in a current alarm
list (GetEventInformation).
● A generic operator unit learns at start up with a broadcast who the other participants are.
● A BACnet object with a broadcast (WhoHas) is searching for another object with a unique name, e.g.
B01’Flr1’RS100’HclHwDmd.
(BACnet) broadcasts on a large IP network would take down the entire network.
Layer 3 switches (IP router) block broadcasts to prevent this.
BBMD functionality is available in BACnet to nevertheless allow the use of BACnet broadcasts over
multiple IP segments.
A BBMD (BACnet Broadcast Management Device) is set up on each IP segment. The BBMDs recognize
one another through a table.
The applicable BBMD recognizes a broadcast triggered in a segment and distributes it as Unicast in a
targeted manner to all other BBMDs that are entered in the Broadcast Distribution Table BDT of BBMDs.
The broadcast is forward in other segments using their BBMDs.
Only one BBMD per IP segment is permitted. One BBMD must have a fixed IP
address and use a UDP port. BBMD is not a separate device, but rather
auxiliary functionality on a BACnet device.
In the use cases to date (Desigo PX in multiple IP segments with the same UDP port) a symmetrical BDT is
always used. In other words, all BBMDs are entered in the Broadcast Distribution Table and distributed in
this manner over the bypass to all other BBMDs in the various IP segments. Mid-sized to large Desigo
BBMD
BBMD
BACnet Broadcast
q
Layer 3 Switch
196.168.2.x 196.168.1.x
Layer 3 Switch
196.168.2.x 196.168.1.x
No BACnet Broadcast
w