KNX is a standardized (EN 50090, ISO/IEC 14543) network communications protocol for intelligent buildings. It provides a common way to manage air-conditioning, illumination and other home or office systems.

KNX is the successor of three previous standards: the European Home Systems Protocol (EHS), BâtiBUS, and the European Installation Bus (EIB or Instabus). The standard is based on the communication stack of EIB but extended with the physical layers, configuration modes and application experience of BâtiBUS and EHS.

Device Structure

There are two types of KNX devices: system components (such as power supply unit, line coupler, repeaters, serial interfaces) and end devices (sensors and actuators). All KNX devices have the following structure

• Bus Coupling Unit (BCU): It establishes connection between the bus cable and the application module.
• Physical External Interface (PEI):
• Application Module (AM): Adapted to the respective purpose and contains for example the operating elements of a switch sensor or the relay and supply terminals of a switch actuator.
• Application Software: Software drivers, provided by the device manufacturers which are downloaded to the Application Module using the ETS software (check the ETS section below).

In certain models, the bus coupling unit (BCU) and application module (AM) are permanently linked together.

Transmission Media

The KNX protocol supports several transmission media:

• Twisted Pair Cabling (TP) using a SELV network and supply system. Main characteristics are: data and power transmission with one pair (devices with limited power consumption may be fed by the bus), and asynchronous character oriented data transfer and half duplex bi-directional communication. TP 1 transmission rate is 9600 bit/s.
• Powerline (PL) enables communication over the mains supply network. Main characteristics are: spread frequency shift keying signaling, asynchronous transmission of data packets and half duplex bi-directional communication. PL 110 uses the central frequency 110 kHZ and has a data rate of 1200 bit/s.
• Radio Frequency enables communication via radio signals in the 868,3 MHz band for Short Range Devices. Main characteristics are: Frequency Shift Keying, maximum duty cycle of 1%, 32 768 cps, Manchester data encoding.

Beyond these media, KNX has unified service and integration solutions for IP-enabled media like Ethernet (IEEE 802.2), Bluetooth, WiFi/Wireless LAN (IEEE 802.11), "FireWire" (IEEE 1394) etc.

The communication principles of the protocol are the same for all transmission media:

Communication Principles

The communication principles of KNX do not depend on the transmission medium that is being used.

Datapoints

The information which is passed from one device to another is represented by KNX Datapoints. Datapoints represent device state, parameters, diagnostic data etc. The different Datapoint Types are standardized by the KNX Association. Datapoints are transferred via Group Addresses.

Flags

Each Datapoint has a list of flags that define the communication with the bus:

• Communication (C) - Activates or deactivates the communication of the object. If the C flag is not active, no communication is possible with the bus regardless of the other flag settings.
• Read (R) - Set if the value of the communication object should be read by another bus device.
• Write (W) - Set if the value of the object is modified via a KNX telegram. In general, it is set in actuators.
• Transmit (T) - Set for a sensor so that the modified object value can be transferred on the bus.
• Update (U) - Tells the device if a reply telegram (to a read request) should be treated the same way as a write telegram and modify the internal value of the communication object.

Interworking Datapoint Types (EIS Types)

The Datapoint Types are defined as a combination of a data type (defined by format, encoding) and a dimension (defined by range and unit). They are identified by a 16 bit main number separated by a dot from a 16-bit subnumber, e.g. "7.002". The main number (left) stands for format and Encoding, while the subnumber (right) stands for Range and Unit.

Datapoint Types with the same main number thus have the same format and encoding. A different subnumber indicates a difference in range and/or unit.

Group Addresses

Information (represented as Datapoints) is passed from one device to another via Group Addresses. The Group Address identifies one or several entities in the network (one Shared Variable or several devices) which connects two Datapoints together in the same way as the hook-up wire links the push button and the impulse switch in the conventional push button circuit.

Group Addresses are defined globally for the whole network during the time the network is configured.

ETS

The Engineering Tool Software (ETS) is a Windows program for KNX configuration, which is actually a part of the KNX standard. It allows the developer to configure the KNX network by importing drivers for KNX devices, assigning Group Addresses etc.

ETS can export data about the network in XML format, which can then be imported by third party applications and used for controlling KNX devices (check the Configuring the KNX Module for Management via the Home Device Manager section in the Setup Guide).