KNX Home Automation: Basics, Protocol, and Frame Structure

This article covers the basics of KNX technology, including KNX RF PHY layer features, frame structure, and addressing scheme. KNX home automation is used in the IoT (Internet of Things). An alliance of over 300 companies from 34 countries has agreed on uniform telegram traffic between sensors and actuators in a bus system, leading to the development of EIB (European Installation Bus). KNX standard is the successor to EHS, BatiBus, and EIB.

Communication between KNX devices is done using group addresses, and it’s widely used in the home and residential market.

There are various versions of KNX, depending on the communication medium:

• KNX IP: KNX bus communication via Internet Protocol
• KNX RF: KNX bus communication via radio frequency (RF)
• KNX TP: KNX bus communication via 2-wire link (twisted pair)
• KNX over powerline
• KNX over infrared
• KNX over Ethernet

KNX Standards

• International Standard: ISO/IEC 14543-3
• European Standard: CEN ELEC EN50090
• Chinese Standard: GB/T 20965
• US Standard: ANSI/ASHRAE 135

KNX Addressing Scheme

A KNX system consists of sensors and actuators. Retransmitter devices are used for larger installations. Both bidirectional and unidirectional implementations are possible in a KNX system.

The system uses two types of addressing schemes for communication:

• Group Addressing: Used for multicast and broadcast transmissions. Addresses are stored in the “Address Table.”
• Individual Addressing: Used for point-to-point communication. It has a 16-bit device address, also stored in the “Address Table.”

In powerline communication, a 48-bit domain address is added to each message.

KNX Features as per KNX RF Physical Layer (PHY)

Table 1 outlines the features of the KNX RF physical layer.

Table-1: KNX RF Physical Layer features

KNX Frame Structure

Fig-1: KNX frame structure

The figure above shows the KNX frame structure as per IEC870-5-2 (FT3). It starts with a preamble and ends with a postamble. Each data block is appended with CRC for error detection. Both preamble and postamble are used for synchronization.

• Block1 = { Length(1byte), C-field(1byte), ESC(1byte), Ctrl(1byte),SN(6bytes)}

  • Length: Total number of user bytes counting from C-field
  • C-field: Fixed value of 44h
  • ESC: Escape code to separate KNX from metering
  • Ctrl: Uni or bidirectional sender signal strength battery state
  • SN: Serial number or domain address of the sender

• Block2= {KNX-Ctrl(1byte), Standard KNX frame(variable),Data(Variable)}

  • KNX-Ctrl: Standard or Extended

KNX Protocol Stack

The KNX protocol stack consists of the physical layer, data link layer, routing or network layer, transport layer, and application layer.

KNX Home Automation Basics

Fig-2: KNX home automation

KNX is widely used in building and home automation for smart homes and cities. It has numerous applications for energy-efficient networks in schools, houses, buildings, and more. Companies like Siemens and Schneider Electric are actively involved in projects using KNX technology.

KNX is a mature protocol for wired media and is recognized internationally. To take advantage of both wired and wireless mediums, KNX is often used in conjunction with other wireless technologies (Zigbee, Z-Wave, etc.) for home automation. The figure depicts one such network using a KNX-Zigbee Gateway. This gateway facilitates protocol conversion between KNX and Zigbee. Wired devices are managed by KNX protocols, while wireless devices are managed by Zigbee protocols.