Beckhoff: Get ready for the next automation revolution
Industrial Ethernet Book Issue 69 / 35
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Building automation bus design

The automation of buildings has been accelerating over the past two decades. There are various aims, the main ones being to make buildings secure, save energy and control their climate to maximise occupant comfort and safety. KNX building automation technology has millions of installed nodes across the world. Over 250 KNX member companies worldwide offer around 7000 certified product groups in their catalogues, from different application domains. Heinz Lux of the KNX Association provides a flavour of building automation techniques, and how a typical KNX bus can be designed and implemented.

Fig. 1: KNX is suitable for new and existing buildings, large and small. Interlinking with a single bus considerably reduces design and installation time, and the installation is easily adapted to new applications and is extendable.

THE DEMAND for comfort and versatility in the management of air-conditioning, lighting and access control systems is growing fast for factories, public and retail buildings, offices and even domestic homes. Moreover, the efficient use of energy is becoming crucial.

However, greater convenience and safety coupled with lower energy consumption can only be achieved using intelligent control and monitoring of all involved products and systems. Previously, this meant installing ever more complicated wiring, running from sensors and actuators to the control and monitoring centres. Such masses of wiring led to high design and installation effort, increased fire risk and soaring costs. Just as in industrial automation, the answer was to ensure that all components communicated via a single common language.

There are a number of such building automation languages, but probably the most important is the manufacturer and application domains-independent KNX Bus, which is administered by the KNX Association. KNX, which is three previous standards combined - the European Home Systems Protocol (EHS), BatiBUS, and the European Installation Bus (EIB) - is a standardised (EN 50090, ISO/IEC 14543) OSI-based network communications protocol designed for intelligent buildings. KNX is based on the communication stack of EIB, but has been enlarged with the physical layers, configuration modes and application experiences of EHS and BatiBUS. KNX installations can be planned and configured by a common multi-vendor and application-independent PC tool, the ETS. The newest version is ETS4, which is backwards compatible to ETS3 and ETS2.

Bus devices are typically the sensors or actuators that control building management equipment. Via the KNX medium to which all bus devices are connected (twisted pair, radio frequency, power line or IP/Ethernet), these devices exchange information. Such interlinking with a single bus considerably reduces design and installation time. Furthermore, such an installation is easily adapted to new applications and is extendable - just as is typically the case with industrial networking systems. It is, therefore, suitable for new as well as existing buildings, large and small.

Configuration modes

The standard includes two different configuration modes:

• S-Mode (System Mode) - This highly flexible configuration mechanism allows well-trained installers to build sophisticated building control functions. An installation comprising S-Mode components can be planned by a common software tool (ETS 4 Professional) on the basis of product databases provided by SMode product manufacturers. The ETS is also used to link the products and configure them.

• E-Mode (Easy Mode) - For installers having only basic training, E-Mode compatible products offer limited functions compared with S-Mode, and components are already pre-programmed and loaded with a default set of parameters. With a simple configurator, each component (mainly its parameter settings and communication links) can be partly reconfigured.

Communication media

The standard includes several communication media, each of which can be used in combination with one or more configuration modes, so each manufacturer can choose the right combination for specific applications. These media are:

• TP1 (Twisted Pair) - This, with its bitrate of 9600bps, has been taken over from EIB, so EIB and KNX TP1 certified TP1 products will communicate with each other on the same busline.

• PL (Powerline - PL110) - With a bit rate of 1200bps, this has also been taken over from EIB, so EIB and KNX PL110 certified products will operate and communicate with each other on the same electrical distribution network.

• RF (radio frequency) - Devices supporting this wireless medium transmit KNX telegrams in the 868MHz (Short Range Devices) frequency band, with a maximum radiated power of 25mW and bitrate of 16,384kbps. It can be developed using off-the-shelf components, and it allows uni- and bidirectional implementations, has a low power consumption and - for small and medium size installations - re-transmitters are needed only in exceptional cases.

• IP (Ethernet) - As documented in KNXnet/IP specifications, telegrams can also be transmitted encapsulated in IP telegrams, so LAN networks and the Internet can route or tunnel KNX telegrams. IP routers are an alternative to USB data interfaces or for TP line or backbone couplers. In the latter case, the TP backbone is replaced by a fast Ethernet-based line.

Fig. 2: For lighting control. KNX systems can be mapped to BACnet objects (ISO 16484-5) or can interface with lighting control technologies (such as DALI).

Several KNX manufacturers offer gateways to other building automation systems, telephone networks, multimedia networks and IP networks, etc. KNX systems can be mapped to BACnet objects (ISO 16484-5) or can interface with the DALI lighting control technology (Fig. 2). A typical application - smart metering - is shown in Figure 3 (below).

Fig. 3: Smart metering. This shows a typical building automation application, which is the use of smart meters to provide energy users and utilties with proper energy use and billing information.


• ETS 4 - This is the multi-vendor and application independent KNX design and commissioning tool. It is available in an unlimited version (ETS Professional), limited version (ETS Lite) and a free of charge version (ETS Demo). The ETS Professional and the ETS Lite can be used PC dependant, or with the help of a USB dongle - also PC independent.

• iETS Server - This is a gateway between KNX and IP that allows users to connect to KNX networks through the LAN or Internet. It provides assistance for installers, service providers, facility managers, building owners or users for monitoring and trouble shooting, upgrades and more - remotely if required. Using the EIBlib/IP protocol (not KNXnet/IP), it allows iETS clients (such as ETS 3) to connect to the network. With iETS, the person technically responsible for an installation may use the ETS to access the system remotely. The server allows remote programming or diagnostics via IP (Fig. 4). The KNX Association offers the iETS Server as a Windows program, while some member companies offer it as KNX certified devices.

Fig. 4: Screenshot from a KNX system running version ETS4. Highly flexible configuration mechanisms allow welltrained installers to build sophisticated building control functions relatively and easily.

• KNX Falcon SDK - The Falcon Driver Library provides access management for bus devices, for telegrams (group address), physical addresses, device states and much more. It is a DCOM-based (Microsoft Distributed Component Object Model) library for Windows and it can be used across the LAN. It offers an application program interface (API) for sending and receiving telegrams across the network. Access is supported through RS 232, USB and the Internet. The Library allows the programming of languages including Visual C++, Delphi, Visual Basic and others, and is the network connection library for ETS 3 and EITT. It also offers IConectionManager, a simple user interface for the integrator to make and configure connections between KNX and other media.

• EITT - This is a special analysis Interworking Test Tool for putting devices through their paces. It is mainly used by manufacturers and test laboratories for testing, troubleshooting and monitoring the Interworking and system stack compliance of KNX products. It also provides analysis and simulation of the device network protocol. The EITT supports tests through two COM interfaces simultaneously. Telegrams are recorded online and can be analysed via a multitude of filter criteria. Various trigger functions are available. In addition, the EITT can send telegram sequences for simulation and test purposes. States, such as ACK, NAK, BUSY or Flags, are shown in the bus monitor.

Structure and implementation

What follows describes how an installation can be configured. It is not meant as a guide; merely to provide a flavour of how KNX can be implemented in practise.

While a topology is a description of the structure and hierarchy of a network, including the devices and communication paths, the Logical Topology describes the configuration of an installation from the point of view of the communication flow. The Physical Topology gives a - mostly simplified - description of the physical layout of the communication media and the physical location of KNX devices in it. Figure 5 shows the KNX model schematically.

Fig. 5: KNX model overview. KNX specifies many mechanisms to bring the network into operation, while enabling manufacturers to choose the most adapted configuration for their chosen market.

Medium independent

The logical topology of a KNX installation is structured in a Backbone Line, Main Lines and Lines. This structure is reflected in the individual addressing of KNX devices. There is one single Backbone Line and Main Lines are hierarchically subordinate to it. Up to 15 Main Lines can be defined in an installation (numbered 1 to 15). The Backbone Line has Main Line number 0. No Main Line may have Main Line number 0.

Main Lines can be connected to Backbone Lines using a backbone coupler, which always has device number 0 and Line number 0 (only one/Main Line maximum).

Lines are hierarchically subordinate to the Main Lines, and up to 15 (numbered 1 to 15) of them can be defined per Main Line in an installation. A Main Line must always have Line Number 0 (no Line can be numbered 0). Line numbers are written preceded by the number of the Main Line that the Line is connected to.

Here are some examples: '4.9'may be the notation for the Line 9 connected to Main Line 4. Lines can be connected to Main Lines using a line coupler - at most one per Line. The line coupler always has the device number 0; no other device can be numbered 0.

An Area comprises the whole of a Main Line with subordinate Lines and all connected devices. An installation may, therefore, count up to 15 Areas. The Area number is the number of the Main Line it contains, but the Backbone Line with the devices attached to it is not seen as an Area.

A Subnetwork is any part of this topology with the same Main Line number and the same Line number. The Backbone Line, any Main Line and any Line are subnetworks.

KNX end devices can be connected anywhere in this topology, and up to 255 of them can be addressed in any subnetwork. End devices are numbered from 1 to 255, but note that end devices must not be numbered 0.

Hierarchical structure

The Logical Topology defines a hierarchical organisation of the KNX Network, in that messages can only pass from one subnetwork to another following a path in the Logical Topology. Every KNX device (backbone coupler, line coupler, end device etc) must have an individual address that is unique throughout the topology.

Looking at reserved individual addresses, device number '0' is reserved for backbone couplers and line couplers. Note that as a backbone coupler is installed in a main line, its line number is also '0'. The following gives some examples of the Logical Topology reflected int he individual addresses:

'4.9.12' may be the individual address for any KNX end device.

'4.9.0' is the individual address of the line coupler that couples Line 4.9 to Main Line 4.

'7.0.0' is the individual address of the backbone coupler coupling Main Line 7 to the Backbone Line.

Media in the Logical Topology

The subnetwork is the smallest part of the Logical Topology that can be implemented with one single physical layer type. Splitting a subnetwork into different physical layer types is not allowed.

A Domain is that part of the Logical Topology over which the data signals of one physical layer type propagate, possibly with re-sending having identical characteristics, but without changing the message contents and with respect of the Logical Topology. The smallest possible domain is, therefore, a subnetwork. A larger domain may enclose one or more united subnetworks.

Closed media are the physical layer types on which the message signals propagate along the medium in a controllable way. A twisted pair is an example of this. Open Media are the physical layer types on which the message signals do not follow a wire-like physical medium. Examples include powerline and radio frequency.

In open media, it may be impossible to physically separate the following:

Subnetworks - Here, the backbone and line couplers are optional. Lines can then be connected to Main Lines without a line coupler, and Main Lines may be connected to the Backbone Line without a backbone coupler.

Domains - If identical transmission characteristics are used in two domains, messages pass undetected from one domain to another. To allow separating subnetworks and Domains - and so respect the Logical Topology - the Link Layers of open media limit their communication by using a domain address. This separates between Domains in one KNX network and between different KNX networks. The domain address is an unstructured 16-bit number. Every domain has an own unique domain address within an installation. Domain address 0000h is reserved for cross-media broadcast communication. Figure 6 shows examples of domains and domain addresses.

Fig. 6: Examples of domains and domain addresses. Layers of open media limit their communication by using a domain address, which separates between domains in one KNX network and between different KNX networks.

KNX applications

• Lighting control (switching, dimming, mood lighting)

• Shading control (shutters, blinds)

• HVAC (individual room temperature control, control of radiators, thermal units, boilers, coolers, fans etc)

• Access & security (presence detection, burglary and fire detection and alarms, presence simulation, panic switch)

• Energy management (consumption metering, load shedding etc)

• Comfort functions and intelligent control across all applications (central user control, combined scenarios, intelligent process control)

• Remote control and remote maintenance (e.g. via phone or Internet)

• Interfacing with complementary or peripheral systems (white goods, supervision consoles, facility management, dedicated security systems, audio, multimedia and services etc).

Message routing

To allow for connection oriented communication between any two devices (possibly connected to different media or with different media in-between), the individual address must be unique throughout all used media.

If neighbouring subnetworks are connected to each other via a line- or backbone-coupler - a media coupler must be used. For coupling different media, this coupler must filter messages in both directions. This routing is a functionality of the network layer within the coupler. The following cases are possible:

Destination address = group address - The routing here is based on a routing table containing information for group addressed messages.

Destination address = individual address - The routing is, in this case, based on a comparison between the coupler's individual address and the destination address.

Destination address = broadcast

• Closed media - messages are always passed in both directions.

• Open media - messages may be passed (or not), according to the domain address:

Domain address 0x0000 - The media coupler does not pass the message to the other medium.

Domain address = 0x0000 - The media coupler passes the message to the other medium.


KNX is approved as an international standard (ISO/IEC 14543-3), as well as a European standard (CENELEC EN 50090 and CEN EN 13321-1), the US ANSI/ASHRAE 135, and the Chinese GB/Z 20965 standard.

Over the past few years, the use of KNX in domestic homes has increased dramatically by over 55%. Additionally, for example, KNX member company Cisco Systems is connecting with KNX for smart connected building solutions, and even the smart grid. These are developments that will be watched, because the business opportunities are potentially huge.

Heinz Lux is director of sales and marketing at the KNX Association.

Source: Industrial Ethernet Book Issue 69 / 35
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