WEINZIERL ENGINEERING GmbH
2008-09-08
Seite 1/5
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.