KNX (standard)


KNX is an open standard for commercial and domestic building automation. KNX devices can manage lighting, blinds and shutters, HVAC, security systems, energy management, audio video, white goods, displays, remote control, etc. KNX evolved from three earlier standards; the European Home Systems Protocol, BatiBUS, and the European Installation Bus. It can use twisted pair, powerline, RF, or IP links. On this network, the devices form distributed applications and tight interaction is possible. This is implemented via interworking models with standardised datapoint types and objects, modelling logical device channels.

KNX standard

The KNX standard has been built on the OSI-based EIB communication stack extended with the physical layers, configuration modes and application experience of BatiBUS and EHS.
KNX installations can use several physical communication media:
KNX is not based on a specific hardware platform and a network can be controlled by anything from an 8-bit microcontroller to a PC, according to the demands of a particular building. The most common form of installation is over twisted pair medium.
KNX is an approved standard by the following organisations, :
It is administered by the KNX Association cvba, a non-profit organisation governed by Belgian law which was formed in 1999. The KNX Association had 443 registered hardware and software vendor members from 44 nations as at 1 July 2018. It had partnership agreements with over 77,000 installer companies in 163 countries and more than 440 registered training centres. This is a royalty-free open standard and thus access to the KNX specifications is unrestricted.

KNX architectureAll information in this and subsequent sections is summarised from

KNX devices are commonly connected by a twisted pair bus and can be modified from a controller. The bus is routed in parallel to the electrical power supply to all devices and systems on the network linking:
Classifying devices as either "sensor" or "actuator" is outdated and simplistic. Many actuators include controller functionality, but also sensor functionality.
Application software, together with system topology and commissioning software, is loaded onto the devices via a system interface component. Installed systems can be accessed via LAN, point to point links, or phone networks for central or distributed control of the system via computers, tablets and touch screens, and smartphones.
The key features of the KNX architecture are:
Central to the KNX architecture concepts are datapoints which represent process and control variables in the system. The standardised containers for these datapoints are group objects and interface object properties. The communication system offers a reduced instruction set to read and write datapoint values. Datapoints have to confirm to standardised datapoint types, themselves grouped into functional blocks. These functional blocks and datapoint types are related to applications fields, but some of them are of general use. Datapoints may be accessed through unicast or multicast mechanisms.
To logically link applications datapoints across the network, KNX has three underlying binding schemes: one for free, one for structured and one for tagged binding:
The common kernel sits on top of the physical layers and the medium-specific data link layer and is shared by all the devices on the KNX Network. It is OSI 7-layer model compliant:
An installation has to be configured at the network topology level and at individual nodes or devices. The first level is a precondition or “bootstrap” phase, prior to the configuration of the distributed applications, i.e. binding and parameter setting. Configuration may be achieved through a combination of local activity on the devices, and active network management communication over the bus.
The KNX configuration mode:
Some modes require more active management over the bus, whereas some others are mainly oriented towards local configuration. There are three categories of KNX devices:
KNX encompasses tools for project engineering tasks such as linking a series of individual devices into a functioning installation and integrating different media and configuration modes. This is embodied in an Engineering Tool Software suite.

Devices

A KNX installation always consists of a set of devices connected to the bus or network. Device models vary according to node roles, capabilities, management features and configuration modes, and are all laid down in the profiles. There are also general-purpose device models, such as for bus coupling units or bus interface modules.
Devices may be identified and subsequently accessed throughout the network either by their individual address, or by their unique serial number, depending on the configuration mode. Devices can also disclose both a manufacturer specific reference and functional information when queried.

Logical topology and individual address space

A KNX wired network can be formed with tree, line and star topologies, which can be mixed as needed; ring topologies are not supported. A tree topology is recommended for a large installation.
KNX can link up to 57,375 devices using 16-bit addresses.
Coupling units allow address filtering which helps to improve performance given the limited bus signal speed. An installation based on KNXnet/IP allows the integration of KNX sub networks via IP as the KNX address structure is similar to an IP address.

Physical transmission media

TP 1

The TP1 twisted pair bus provides asynchronous, character oriented data transfer and half-duplex bidirectional differential signaling with a signaling speed of 9600 bit/s. Media access control is via CSMA/CA. Every bus user has equal data transmission rights and data is exchanged directly between bus users. SELV power is distributed via the same pair for low-power devices. A deprecated specification, TP0, running at a slower signalling speed of, has been retained from the BatiBUS standard but KNX products cannot exchange information with BatiBUS devices.

PL 110

PL 110 power-line transmission is delivered using spread frequency shift keying signalling with asynchronous transmission of data packets and half duplex bi-directional communication. It uses the central frequency 110 kHz and has a data rate of 1200 bit/s. It also uses CSMA. KNX Powerline is aimed at smart white goods, but the take-up has been low. An alternative variant, PL 132, has a carrier frequency centred on 132.5kHz.

RF

RF enables communication in the 868.3 MHz band for using frequency shift keying with Manchester data encoding.

IP

KNXnet/IP has integration solutions for IP-enabled media like Ethernet, Bluetooth, WiFi/Wireless LAN, FireWire etc.

Frame (telegram) overview

Ignoring any preamble for medium-specific access and collision control, a frame format is generally:
OctetRole
0control field
1 - 2source address
3 - 4destination address
5address type | NPCI | length
6 - 7Transport Layer Protocol Information | Application Layer Protocol Information | data/APCI
8 - N-1data
N ≤ 22frame check

Security

KNX Telegrams can be signed or encrypted thanks to the extension of the protocol that was developed starting in 2013, KNX Data Secure for securing telegrams on the traditional KNX media TP and RF and KNX IP Secure for securing KNX telegrams tunnelled via IP. KNX Data Secure became an EN standard in 2018, KNX IP Secure an ISO standard in 2019.

Conformity

Any product labeled with the KNX trademark must be certified to conform with the standards by accredited third party test labs.