WEINZIERL ENGINEERING GmbH

2008-09-08

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WEINZIERL ENGINEERING GmbH

“KNX IP only” – A New Class of KNX Devices

WEINZIERL ENGINEERING GmbH

Dr.-Ing. Th. Weinzierl

D-84558 Tyrlaching

www.weinzierl.de

“KNX IP only” means communication only over the Internet Protocol in a computer network. At

first this may appear to be a replacement of the proven medium for building systems, but in fact

it’s an extension of the KNX system which will allow an entirely new class of KNX devices in

the future.

IP will be treated as a stand-alone media within KNX and will be considered equal to Twisted

Pair, Powerline and Radio Frequency (RF). It opens doors to top level communication within a

building (e.g. telecommunication, multimedia, etc.) and allows external access to a KNX

installation.

Figure 1: Ethernet Patch Wire

Communication via KNXnet/IP

The transmission of KNX messages via Ethernet is defined as part of the KNXnet/IP protocol

and is already part of the KNX standard. The first application of the KNXnet/IP protocol was to

utilize Ethernet for PC interfaces and routers. IP routers are comparable to line couplers and use

Ethernet as the main line. In the meantime IP has been integrated as stand-alone medium within

KNX and allows using the powerful KNX feature, like the configuration modes and

interworking, also on IP.

Configuration

The proven configuration modes of the existing KNX media are also available for KNX IP

devices. Since KNX IP devices are expected to be fairly complex, most of these devices will be

limited to the configuration in System Mode via the ETS. KNX IP devices will be supported by

ETS.

“KNX IP only” – A New Class of KNX Devices

WEINZIERL ENGINEERING GmbH

2008-09-08

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The accustomed ETS presentation with group objects and parameters will of course be carried

over to the KNX IP device class. In Easy Mode and even in Easy Push Button Mode interesting

solutions based on KNXnet/IP can be expected.

Interworking

The interworking on KNXnet/IP is unchanged from the other KNX media. Therefore, KNX IP

devices may utilize the proven performance of the KNX architecture including the group

addressing. In addition, IP only devices can be plugged to other KNX media easily over IP-

Router.

Protocol

Version

Service Type

Identifier

Total

Length

Header

Ethernet

Header

UDP

Header

IP

KNXnet/IP

KNX-Telegram (cEMI)

Ctrl2

Src hi

Src lo

Dst hi

Dst lo

L

TPCI

APCI

Data

...

0xE0

0x11

0x01

0x00

0x01

0x03

0x00

0x80

Ctrl1

0xBC

AddIL

0x00

MC

0x29

Payload

Header

Length

...

Figure 2: Message format KNXnet/IP

Ethernet as Competitor to TP?

With the growing importance of communication via IP and Ethernet, the question arises as to

whether Ethernet will replace the proven KNX Media Twisted Pair and Powerline entirely. This

question can clearly be answered, „NO“. The basic reason for this is the high cost of having to

wire each end device with its own network cable.

However, this is not a disadvantage if a device already has to have a network connection due to

its application. For example, this will enable the use of buttons on a VoIP telephone for KNX

functions (e.g. dimming of lights) if the manufacturer supports this in the firmware. This means

that every device with a network connection could be turned into a KNX device without

additional hardware costs through the integration of system software for KNX.

The future therefore clearly belongs to hierarchical topologies: Ethernet will establish itself as

the high performance backbone and connection for complex (KNX IP) devices. Twisted Pair,

Powerline and RF will claim their importance in connecting distributed sensors and actuators.

“KNX IP only” – A New Class of KNX Devices

WEINZIERL ENGINEERING GmbH

2008-09-08

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LAN-Switch

KNX TP

1.1.1

1.1.2

1.1.11

1.1.10

KNX IP

Router

1.1.0

KNX IP

Router

2.1.0

KNX TP

2.1.1

2.1.2

2.1.11

2.1.10

Internet

ETS

KNX

IP only
Device

5.1.1

Figure 3: Mixed bus topology with IP and Twisted Pair

Hardware Aspects for KNX IP

The development of KNX IP devices does not require special KNX parts. An IP node basically

consists of an Ethernet controller and a micro controller or a CPU. Ethernet controllers are

available from different semiconductor manufacturers. Most of the time controllers with a data

rate of 10 MBit/s are sufficient.

The choice of the microcontroller basically depends on the required computing power of the

device. In principle, the KNXnet/IP protocol could even be implemented on an 8-bit controller.

Depending on the application higher performing controllers may be necessary. Numerous

controllers already offer an interface for Ethernet on the chip, so that only the physical layer has

to be added. PC platforms for may be interesting for higher level requirements to the

performance.

Figure 4: Platform for a KNX IP only device

IP only devices will commonly have higher energy consumption than typical devices on KNX

Twisted Pair. Depending on the application these will be equipped with dedicated power supply

“KNX IP only” – A New Class of KNX Devices

WEINZIERL ENGINEERING GmbH

2008-09-08

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(230 V) or external power adapter. The alternative power supply via Ethernet Standard Power-

over-Ethernet (PoE, IEEE 802.3af) may be especially interesting. KNX IP devices can be

supplied with power over the Ethernet cable of up to 15 W if the terminal device and the utilized

network switch support this standard.

System Software for KNX IP Devices

The system software for a KNX IP device actually consists of two protocol stacks. An IP stack

with UDP (User Datagram Protocol) is necessary for the communication over Ethernet because

KNX IP is based on connectionless communication. Singlecast and Multicast message are

utilized over UDP.

There is a large choice of commercial and open source software providers for IP stacks. Many

operating systems, for example Linux or Windows CE® already contains the IPstack.

The actual KNX stack is piled upon the IP/UDP stack. The KNX stack uses the IP/UPD stack as

interface to the network. The embedding of KNX messages in UDP messages is defined by

KNXnet/IP. The KNX stack not only contains protocol layers based on the OSI/ISO reference

model but also the particular device model with corresponding management procedures.

The device model 5705 was the first to be defined for KNX IP. It is similar to the device model

0705 from Twisted Pair. It allows up to 255 group objects and supports a loadable area for

several thousand parameters.

The application program in the device accesses the API (Application Programming Interface) of

the KNX stack to communicate in the KNX network on the level of group objects.

KNX Msg.

Ethernet

IP/UDP

KNXnet/IP

KNX Stack

Application

Figure 5: System software of a KNX IP device

“KNX IP only” – A New Class of KNX Devices

WEINZIERL ENGINEERING GmbH

2008-09-08

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Weinzierl Engineering GmbH (www.weinzierl.de) as system provider offers different solutions

to efficiently develop and manufacture KNX IP devices. Based on detailed experience in the

KNX development and certification new system software (communication stack) for KNXnet/IP

was implemented. The bus monitor Net’n Node now also supports the additional interfaces

KNXnet/IP Tunneling and Routing.

The KNX stack for KNX IP devices can be used with any IP stack, e.g. that of Linux. In addition

an own IP stack is available which has been specifically developed for small platforms without

an own operating system.

Summary

By the very definition of a KNX IP device, KNX now offers the complete set of media for

building control. KNXnet/IP bridges digital communication systems with high band width like

audio, Internet and telecommunication.

KNXnet/IP offers companies which are already involved in KNX new possibilities for the

development of trend setting products. The complete availability of hardware and software

allows manufacturers a quick and cost effective integration of their devices into the world of

KNX. The combination of two world standards - Ethernet and KNX - is an equally safe basis for

manufacturers and installers for investments in new solutions.