User Manual

System configuration
Technical limits and limit values
22
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A PX site is independent of the limits affecting the BACnet network. A site can extend over several BACnet
networks. One BACnet network may include several sites. All the associated limits must be maintained
simultaneously.
A PX site cannot be extended beyond the limits of a BACnet internetwork. This is particularly important in
the case of BACnet PTP internetworks.
PX plant
A PX plant is part of a PX site and generally comprises several partial plants (plant structure).
A PX plant can be distributed over several PX BACnet devices. In principle PX BACnet devices can be
distributed to different BACnet networks. However, owing to the communications load between partial
plants, this is not recommended.
The plant structure is mapped to BACnet by means of hierarchy objects. Operator units with generic
operation automatically read this structure.
BACnet MS/TP
A BACnet MS/TP network is a BACnet network that is physically based on EIA-485 and operated using a
BACnet-specific MasterSlave/TokenPassing data link protocol (see BACnet standard clause 9). An MS/TP
network is linked via a BACnet router to a BACnet/IP or BACnet/LonTalk network.
Desigo room automation
Includes the BACnet devices connected directly to BACnet/IP or BACnet MS/TP, used for room
automation.
These BACnet devices are not part of a PX site. There is no connection via global objects and the primary
copy procedure.
Desigo room automation system functions
In Desigo room automation, primary subsystem control functions are centralized as Desigo room
automation system functions.
PX system functions
A PXC.. of a PX site as PX system function can assume Desigo room automation subsystem functions such
as scheduling, life check, time synchronization for a Desigo room automation system function group for
BACnet devices for room automation.
System function group
A Desigo room automation system function group cannot be identified or defined via the network topology.
Engineering the Desigo room automation system functions of the PX system functions determines the
Desigo room automation system function group.
For more information, see chapter
System Overview and Network Architecture .
22.1 Technical limits and limit values
There are two types of limits:
● Technical limits (hard-coded limits) are maintained by technical means. They cannot be exceeded.
● Recommended limits (soft limits) are not enforced by technical means. They can be exceeded.
The limits are defined to ensure the full and correct functioning of the system. Consult Headquarters
before exceeding the recommended limits. HQ can modify the recommended limits on the basis of new
findings at any time. Changes are notified in Facts bulletins.
Certain limits cannot be verified (for reasons of cost or quantity). These limits are shown in this document
as follows: