System overview

Desigo system

Covers all the devices on the MLN (Management Level Network), ALN (Automation Level Network) and FLN (Field Level Network).

One Desigo system may comprise several BACnet internetworks. These are connected into a system with Desigo CC. In this case, Desigo CC appears as a BACnet device in several BACnet internetworks.

BACnet internetwork

A BACnet internetwork consists of one or several BACnet networks. Individual BACnet networks are connected to BACnet routers.

Each BACnet device can communicate with another BACnet device in the internetwork. A BACnet device in one internetwork cannot communicate with a device in another internetwork.

A Desigo management station can be used to integrate the operation of several BACnet internetworks and other systems (see Desigo system).

When defining the system configuration, FLN integrations (LonWorks, KNX) are also added to the BACnet internetwork. In this way, the Desigo system can be seen as a combination of several BACnet internetworks. Technically, the individual FLN devices are not BACnet devices. They do not communicate via the BACnet protocol.

BACnet PTP internetwork

BACnet PTP communication uses modem (telephony) or null-modem (RS232) connections. Owing to the slow rate of data transfer via these connections, the limits are lower for a BACnet PTP internetwork. Modem-based PTP connections are considered obsolete and are therefore no longer used. The BACnet PTP communication connects BACnet networks via BACnet half routers.

BACnet network

A quantity of BACnet devices connected within an IP or LonTalk or MS/TP network with specific (that means, the devices are in the same BACnet Broadcast Domain) limits. In the case of the LonTalk or MS/TP network, the limit is physical. In the case of an IP network, the network can be physically the same, but the limit is determined by different UDP ports.

Local communication between two BACnet devices in a BACnet network is not visible in another BACnet network.

IP segment

Sub-area of an IP network. IP segments are connected by IP routers.

In order to ensure that BACnet communications (Broadcasts) can always take place across IP routers, BBMDs (BACnet Broadcast Management Devices) are required. PXG3.L/M and PXC…-E.D over IP can be configured as BBMDs. Individual BACnet devices in an IP segment can register with a BBMD as foreign devices.

LonWorks segment (ALN)

Sub-area of a BACnet/LonTalk network. LonWorks segments are connected by LonWorks routers. In most cases it is not necessary to divide a BACnet/LonTalk network into several LonWorks segments (ALN).

It is not possible to use a LonWorks router because of the restricted length of the data packets. An L-Switch can be used as a router on the ALN.

LonWorks segment (FLN)

Sub-area of a LonWorks network. LonWorks segments are connected by LonWorks routers.

An L-Switch or a LonWorks router can be used as a router on the FLN.

LonWorks trunk (FLN)

Comprises all the devices connected on the FLN side of the PXC00.D/-E.D + PXX-L1…. Consists of one or several LonWorks segments (FLN).

A LonWorks trunk (FLN) is the equivalent of a LonWorks network (FLN).

PX KNX integration

Comprises the integration of KNX devices that are connected on the FLN side of the PXC001.D/-E.D.

PX site

A Desigo PX automation system site.

The PX BACnet devices which control the plant in a PX site are interconnected via the global objects and the primary copy procedure.

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.


A BACnet MS/TP network is a BACnet network that is physically based on EIA-485 and operated using a BACnet-specific MS/TP 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.