Basic Documentation

BACnet networks with MS/TP trunks
MS/TP cabling and topology
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5.2 MS/TP cabling and topology
Physically, an MS/TP bus is a line topology, even if it is logically a ring (due to the token passing principle).
The following applies:
● The MS/TP network may only be implemented within a building.
● BACnet MS/TP networks may only be wired in a line topology. T connections/branches are prohibited.
Stub lines are one exception (up to a maximum of 30 meters).
● In the Desigo system there are actually no pure BACnet /MS/TP networks. BACnet/IP is always used
as the backbone.
For details, see
Desigo Ethernet, TCP/IP, MSTP and BACnet (CM110666) and Desigo room automation
engineering, mounting and installation
(CM111043, section 10 Network topology ).
As a rule a BACnet MS/TP-IP router to the IP network is always required when setting up a Desigo project
with BACnet MS/TP devices. Since both the PXC… for primary plants (in plant scope) and the central
functions (in the building scope) as well as Desigo CC as a BACnet-capable management platform only
communicated over BACnet/IP.
On smaller to mid-sized projects with low complexity, all BACnet/IP devices use the same UPD port; the
BACnet-MS/TP-IP router only connects to the BACnet MS/TP networks.
On mid-sized to large projects at medium to high complexity, the BACnet MS/TP-IP router also takes over
the BBMD functionality.
The following diagram illustrates which devices and functions are located in what scope and how the
communication paths are defined. The floor scope does not require/support any interaction with another
floor scope. Conversely, functions and applications in the building scope do communicate with the floor and
plant scope. The plant scope processes the information (forced and demand signals) from the building
scope.
You can determine that a floor can be a floor or floor segment, i.e. a MS/TP trunk with max. 64 participants
(e.g. max. 64 DXR2.M). The MS/TP trunk can, however, also encompass multiple floors with max. 64
participants, in other words, a floor represents a zone with max. 64 BACnet MS/TP-capable devices/room
automation stations. The following also applies to the MS/TP trunk: There are no central functions with
group master function and rooms/room segments can only communicate within the trunk.
An IP network can include a max. 8 BACnet MS/TP-IP router and max. 250 DXR2.M devices. A separate
device is needed per IP network that takes over the BBMD functionality. Devices implementing central
functions can only communicated via BACnet/IP.
Different BACnet networks are interconnected to one BACnet internetwork that can include a max. 50
BACnet MS/TP-IP routers or max. 2500 DXR2.M devices.
PXC3 DXR2
IP Subnet 192.168.103.x / BAC0
Desigo PX
BBMD
IP Subnet 192.168.102.x
CC Client
CC Server
Desigo CC
System-wide Scope
Foreign Device
BBMD
Plant Scope: Primary Plants
IP Subnet
IP Subnet 192.168.106.x
Desigo PX
BACnet Network 4 (BAC0)
BBMD
Building A
BACnet Internetwork 0 (BAC0)
IP Subnet 192.168.100.x
B
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Building Scope:; Central Functions
n Floor Scope: Flexible Rooms
Desigo CC
BACnet Router
MS/TP
BBMD
n Floor Scope: Flexible Rooms
BACnet Router
MS/TP
Desigo PX
BACnet Network 2 (BAC0)
Layer 3
Switch
IP Subnet 192.168.104.x
BACnet Network 3 (BAC0)
IP Subnet 192.168.105.x
Floor Central Functions
Floor Central Functions
PXC3 DXR2 Desigo PX
CC Client CC Server
Desigo CC
System-wide Scope
Plant Scope: Primary Plants
Desigo PX
Building A BACnet Internetwork 0 (BAC0)
IP Subnet 192.168.100.x
Building Scope: Central Functions
Floor Scope: Flexible Rooms
Desigo CC
BACnet Router
MS/TP
Floor Scope: Flexible Rooms
BACnet Router
MS/TP
Desigo PX
Unmanaged
Layer 2 Switch