KNX Basics
The KNX Standard
Last updated
The KNX Standard
Last updated
The key to making a building “intelligent” is to equip it with networked sensors and actuators.
Conventional methods: to employ a star topology where every node is linked by its own cable to a central distribution board in which the logical relationships are held. (However, the system is constrained by the size of the power distribution board and it is also time-consuming to extend.)
Bus technology: to link all sensors and actuators in the building with a “data cable”, and enable them to share information with each other.
All strong brands in the building installation sector are pushing KNX technology.
Because the technology is standardised, KNX products are all mutually compatible and KNX installations can be easily modified or extended at a later stage.
KNX supports all communication media: TP (Twisted Pair), PL (Powerline), RF (Radio Frequency), and IP(Ethernet).
In situations where the customer wants a large number of different functions, a bus system will be easier, cheaper and less complex than an equivalent conventional installation.
All devices in a KNX system use the same transmission method and are able to exchange data via a common bus network.
Decentralised structure: no central control unit.
If one device fails, the others continue to function.
KNX bus systems can be modified and added to exactly as required.
KNX devices
System devices: power supply, programming interface, etc.
Sensors detect events in the building, issue commands, convert these into telegrams, and send them along the bus network.
Actuators receive telegrams, convert the commands embedded in them in to actions.
Communication media
KNX Twisted Pair (KNX TP): communication via a twisted pair data cable (bus cable).
KNX Powerline (KNX PL): uses the existing 230 V mains network.
KNX Radio Frequency (KNX RF): communication via radio signal.
KNX IP: communication via Ethernet.
A two-core twisted pair data cable (bus cable) is the most common communication medium for KNX installations.
In KNX TP the bus cable supplies all bus devices with both data and power.
Control field: defines the priority of the telegram and whether or not transmission of the telegram was repeated (if the receiver did not respond).
Address field: specifies the Individual Address of the sender and the destination address (Individual Address or Group Address) of the receiver.
Data field: contains the telegram’s payload which can be up to 16 bytes long.
Checksum field: is used for parity checks.
A telegram can only be transmitted if no other telegram is being transmitted at the same time.
To prevent collisions during transmission, the priorities of the various sending devices are regulated.
The device sending the 1 “hears” that a 0 is being transmitted along the bus, and detects the collision. It is obliged to abort its own data transmission and give priority to the other transmission. After the transmission taking priority is complete, the aborted data transmission recommences.
A telegram’s level of priority can be defined in its control field.
If two telegrams have the same level of priority, which telegram is allowed to be sent first is determined by its physical address (0 has priority over 1).
Using the existing electricity cables in a building as the KNX communication medium is a cost-effective way of retrofitting a building with KNX.
The data signals are superimposed onto the mains voltage.
KNX PL telegrams are essentially extended KNX TP telegrams.
Training field: synchronises and sets the levels of senders and receivers.
Preamble fields: indicate the start of transmission, control access to the bus, and are needed to prevent telegrams from colliding.
The third field contains the KNX TP telegram.
System ID field: contains an ID for keeping the signals of different KNX PL systems separate, so that only devices using the same system ID can communicate with one another.
Radio is always an appropriate KNX communication medium in those situations where it is not possible to lay new cables in the building (e.g. for sensors in inaccessible areas).
KNX RF is also particularly suitable for extending existing KNX TP installations.
KNX RF telegrams are made up of several data blocks separated by checksum (CRC) fields.
Data blocks: contain the actual payload as well as bus-specific information for addressing purposes.
The first data block
Control field: contains information about the length of the telegram, the transmission quality, the battery status of battery-operated KNX RF devices, and whether the device is unidirectional.
Serial Number/Domain Address: (in E-Mode,) the serial number is evaluated in the receiver together with the source address of the sender; (in S-Mode,) the domain address serves to keep neighbouring RF systems separate from one another.
Checksum: allows the receiver to determine whether or not a telegram has been sent without error.
The second data block
Individual Source Address: physical address.
Individual Destination Address or Group Address
Payload: the actual information that is to be sent.
The existing network infrastructure in the building can be used for the KNX main and backbone lines.
Buildings can be monitored and controlled via Ethernet from anywhere in the world.
KNX customer installations can be analysed and programmed remotely over the internet.
The KNX system uses two Ethernet communication methods – KNXnet/IP tunneling and KNXnet/IP routing – both of which use the UDP protocol.
Tunneling is used to access the bus from a local network or the internet for purposes of e.g. programming the KNX installation.
Routing is used for exchanging telegrams over an Ethernet network, e.g. to couple two KNX TP systems via Ethernet.
Header Length: is always the same and may change in a later version of the protocol.
Protocol Version: indicates the version of the KNXnet/IP protocol.
Service Type Identifier: indicates the action that is to be carried out.
Total Length: indicates the total length of the KNXnet/IP telegram.
KNXnet/IP-Body: contains the payload.
Tunneling is needed where ETS is to be used to send KNX telegrams in a connection-oriented manner within an IP framework.
Communication always takes place via the IP address of the KNXnet/ IP device that is being used for tunneling.
Routing is needed for the simultaneous, connectionless transmission of KNX telegrams to several participants via a KNXnet/IP router. (group communication)
Routing is used for e.g. coupling TP cables.
A KNXnet/IP router serving as a line coupler for a KNX TP cable will only send a telegram to the IP side if the corresponding group address appears in the filter table of the KNXnet/IP router.
All other KNXnet/IP routers serving as line couplers for other KNX TP lines will only transmit the telegram from the IP side to their KNX TP line provided that the relevant group address appears in the filter tables of the KNXnet/IP routers.
Will the popularity of Ethernet eclipse that of the current most popular, established KNX medium, TP? - NO.
substantial cabling costs
time-consuming in networking
high energy consumption
Ethernet will become increasingly established as a high-performance backbone and a means of connecting complex (KNX IP) devices in the hierarchical topology.
The basic unit of a KNX TP installation is a line, including a KNX power supply and choke, and usually no more than 64 other bus devices.
Line Repeaters can be used to extend a line if more than 64 devices are needed.
Line repeaters count as bus devices in the line.
No more than three repeaters can be operated in parallel in a line, meaning the maximum number of bus devices is 255.
Another way of expanding the installation is to create new lines using Line Couplers.
A Line Coupler will not send a telegram to a line for which it is not destined. It reduces the number of telegrams travelling along each line (by taking advantage of the filter function of the line couplers).
Line Couplers in the main line count as bus devices.
Up to 15 lines can be operated on the main line via Line Couplers to form an area.
The main line can accommodate up to 64 devices.
Up to 15 areas can be added to an area line via Area Couplers to form a complete system.
The area line (also called the backbone) can accommodate up to 64 bus devices.
Every device in a KNX system is assigned a unique, unambiguous number – its Individual Address.
This consists of three numbers separated by dots.
The first number denotes the number of the area.
The second number denotes the number of the line.
The third number is a sequential number indicating the device’s position in the line.
Area/Line Couplers must always be given the number 0 as their sequential number.
Example:
Physical address 1.1.0: line coupler coupling the first line with the main line in the first area.
Physical address 2.3.20: bus device 20 in the third line of the second area.
The basic unit of an installation is a line containing a maximum of 255 devices.
An area is created by coupling 15 KNX PL lines using KNX TP.
The maximum number of areas is eight in KNX PL.
Instead of line couplers, in KNX PL System Couplers (also having filter function) are used.
System couplers (like Area and Line Couplers) are assigned the sequential number 0.
Example:
Individual Address 1.5.0: system coupler coupling the fifth PL line with the TP main line in the first area.
Individual Address 2.3.20: PL bus device with sequential number 20 in the third line of the second area.
It needs to be ensured that neighbouring KNX RF installations cannot interfere with one another.
Telegrams sent by KNX radio transmitters always include the serial number/domain address of the device as a unique identifier. Only those receivers paired with the transmitter are able to process telegrams sent by it.
Media Couplers are used for coupling purposes.
Media couplers are assigned physical addresses corresponding to their position in the system topology.
Example:
Physical address 2.3.20: media coupler with sequential number 20 in the third line of the second area.
A KNXnet/IP router (with an Ethernet port and a KNX TP Connection) is used both as Line Couplers and Area Couplers in KNX IP.
The routers are able to filter telegrams and forward KNX telegrams to other KNXnet/IP routers using the routing method.
Most KNXnet/IP routers also support tunneling, i.e. they can also be used as an IP programming interface for ETS.
KNXnet/IP routers can additionally be used to connect entire separate systems with one another via Ethernet.
KNX IP can also be used to network KNX devices, e.g. KNX displays, with one another.
KNXnet/IP routers (routing) are given the sequential number 0.
KNX IP interfaces (tunneling) can be given any sequential number.
Example:
Individual Address 1.5.0: KNXnet/IP router acting as a line coupler, coupling the fifth line with the main line in the first area.
Individual Address 2.3.20: KNX IP programming interface with sequential number 20 in the third line of the second area.
