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 manager 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.
This is a good point to refer once again to the two system limits to complete the concept:
Command process | Designing a system |
|---|---|
Distribution/commanding of group managers to group member | Restricted to max. 500 group members |
Collect data | Designing a system |
|---|---|
Collecting data from the group members via the group manager | Restricted to max. 250 group members |