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venfree/Wiki/KNX Telegram

KNX Telegram

A KNX telegram is the smallest unit of communication on the bus. It contains sender (physical address), destination address (group address), data value and priority, and is received simultaneously by all devices on the line.

Telegram Structure

A standard telegram consists of: control byte (priority, repeat flag), source address (physical address of sender, 2 bytes), destination address (group address or physical address, 2 bytes), NPCI (network protocol control with hop counter, 1 byte), APDU (payload including APCI code, 1–15 bytes), and FCS (checksum, 1 byte).

The APCI code in the APDU field defines the telegram type: GroupValueWrite (write), GroupValueRead (read request) or GroupValueResponse (response to read request). For normal control, GroupValueWrite is the most common type.

Bus Access and Priorities

KNX TP uses CSMA/CA (Carrier Sense Multiple Access / Collision Avoidance). Each device wishing to send waits until the bus is free, then begins transmitting. In the event of simultaneous transmission, the device with higher priority wins, the other detects the collision and waits.

There are four priority levels: System (highest, for group address programming), Alarm, High and Normal. During normal operation, almost all devices send at Normal priority. Safety and alarm telegrams are sent at Alarm priority.

Key Facts

  • Payload (APDU): max. 14 bytes for standard frames
  • APCI types: Write, Read, Response
  • CSMA/CA: collisions resolved via priority arbitration
  • 4 priorities: System, Alarm, High, Normal
  • No ACK → sender retries up to 3 times
  • All devices on the line receive every telegram

Related Articles

KNX Group Address
A KNX group address (GA) is a logical address that links communication objects (COs) of different devices. Whatever is written to a GA is received by all other COs bound to that GA, the publish-subscribe principle of KNX.
KNX Communication Object (CO)
A communication object (CO) is a data point on a KNX device. It has a type (DPT), a direction (send/receive), and is linked to COs on other devices via group addresses.
KNX Flags – C, R, W, T, U, I
KNX flags control the behaviour of a communication object on the bus. They define whether a CO can send, receive or respond to telegrams, and whether it requests a value on startup.
KNX TP – Twisted Pair
KNX TP is the most widely used transmission medium. A twisted pair simultaneously transmits power (29 V DC SELV) and data (9,600 baud), all devices on a line share the same cable.

Related Functions

KNX Binary Input & Push Button Interface
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KNX Switch Actuator
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