KNX Building Automation Utility

KNX Telegram Decoder

Decode KNX TP telegram hexadecimal bytes into control fields, individual and group addresses, routing counter, NPDU length, TPCI, APCI, application data and frame checksum.

✓ KNX TP ✓ Source Address ✓ Group Address ✓ TPCI ✓ APCI ✓ Checksum

KNX

Telegram Structure Decode

● Ready

KNX Telegram Hex Bytes Enter one complete KNX TP telegram including the final checksum byte. Spaces, commas, colons, hyphens, 0x-prefixed bytes and continuous hexadecimal input are supported.

Checksum rule: for a standard KNX TP frame, XOR all bytes from the first control field through the checksum byte. A valid frame produces 0xFF. Equivalently, the checksum byte equals the one’s complement of the XOR of all preceding telegram bytes.

KNX Telegram Decode Result Decoded

Decoded KNX Telegram —

Frame Type —

Control Field 1 —

Control Field 2 —

Source Address —

Destination —

Destination Type —

Routing Counter —

NPDU Length —

TPCI —

APCI —

Checksum —

Checksum Status —

Priority —

Repeat Flag —

Application Bytes —

Total Bytes —

Application / APDU Bytes

—

Telegram Breakdown

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What Is a KNX Telegram?

KNX is a standardized building-automation system used for lighting, HVAC, shutters, sensors, energy management and other control functions. On KNX TP, devices exchange compact bus telegrams containing control information, individual addresses, destination addresses and application data.

A telegram can address another individual KNX device or a group address used by one or more communication objects.

KNX TP Telegram Structure

Control Field 1
Source Address High
Source Address Low
Destination High
Destination Low
Control Field 2
TPCI / APDU bytes...
Checksum

The exact number of APDU bytes depends on the length information carried in Control Field 2.

KNX Individual Address

A KNX individual address is commonly written in the form Area.Line.Device. It occupies 16 bits in the telegram.

Address bytes:
11 01

Binary structure:

Area   = high nibble of first byte
Line   = low nibble of first byte
Device = second byte

11 01
→ 1.1.1

The calculator applies this format to the source address and to the destination when the telegram indicates individual addressing.

KNX Group Address

When the destination-type bit indicates a group address, the destination bytes are interpreted using the KNX group-address bit layout.

The calculator shows both common three-level and raw 16-bit representations.

Three-level group address:

Main / Middle / Sub

Main:
5 bits

Middle:
3 bits

Sub:
8 bits

KNX Control Field 1

The first control byte includes frame-format and link-layer control information, including repeat behavior and priority bits.

This tool exposes the complete hexadecimal byte and separately reports the priority and repeat flag rather than hiding those bits behind one label.

KNX Priority

The two priority bits in Control Field 1 can represent different KNX telegram priorities.

Bits	Priority
00	System
01	Normal
10	Urgent
11	Low

KNX Control Field 2

The second control field contains the destination address type, routing counter and NPDU length information.

Bit 7
Destination Address Type

0 = Individual address
1 = Group address

Bits 6–4
Routing Counter

Bits 3–0
Length information

KNX Routing Counter

The routing counter limits how many routing steps a telegram can pass through. Routers can decrement the counter as the telegram crosses KNX network segments.

Routing Counter:

3 bits

Possible encoded values:
0 through 7

KNX TPCI

TPCI means Transport Layer Protocol Control Information. It occupies the upper bits of the first transport/application byte and helps distinguish unnumbered, numbered and transport-control communication.

A general raw decoder can extract the TPCI bits reliably even when full higher-layer state is unavailable.

KNX APCI

APCI means Application Layer Protocol Control Information. It identifies the application service carried by the APDU, such as group-value read, response or write.

Common APCI	Meaning
0	GroupValueRead
1	GroupValueResponse
2	GroupValueWrite
3	IndividualAddressWrite
4	IndividualAddressRequest
5	IndividualAddressResponse
6	AdcRead
7	AdcResponse

Additional APCI values exist, and some use extended encoding. The decoder reports the numeric APCI value and common names where safely recognized.

KNX GroupValueRead, Response and Write

Three of the most frequently encountered KNX application operations are GroupValueRead, GroupValueResponse and GroupValueWrite.

GroupValueRead:
requests the current value of a group object

GroupValueResponse:
returns a group object's value

GroupValueWrite:
writes or distributes a new group value

The actual engineering meaning of the data depends on the object’s Datapoint Type, or DPT.

KNX Datapoint Types

A KNX telegram does not itself provide the full engineering definition for an application value. The group address is associated with a Datapoint Type in the KNX project configuration.

Examples:

DPT 1
1-bit boolean / switch

DPT 5
8-bit unsigned/scaled value

DPT 9
2-byte KNX floating point

DPT 12
32-bit unsigned value

DPT 14
32-bit IEEE floating point

This decoder therefore preserves application bytes and does not claim that an unknown value represents temperature, dimming, humidity or another measurement without a known DPT.

KNX TP Checksum

The checksum byte allows corruption to be detected in a KNX TP telegram. A convenient validation method XORs every telegram byte, including the received checksum.

Valid telegram:

byte0 XOR
byte1 XOR
byte2 XOR
...
checksum

= FF

The equivalent checksum-generation method XORs all bytes before the checksum and then applies a one’s complement.

KNX Telegram vs KNXnet/IP

A raw KNX TP telegram is not the same thing as a complete KNXnet/IP packet. KNXnet/IP adds its own IP-level header and tunneling or routing structures around KNX cEMI information.

This calculator focuses on the compact KNX TP telegram bytes themselves rather than decoding UDP/IP encapsulation.

KNX Telegram Decoder FAQs

What is a KNX telegram?

It is a compact message exchanged by KNX devices containing addressing, control and application information.

How is a KNX individual address written?

KNX individual addresses are commonly written as Area.Line.Device, for example 1.1.15.

How is a KNX group address written?

A common three-level representation is Main/Middle/Sub, such as 1/2/15.

What does TPCI mean?

TPCI means Transport Layer Protocol Control Information.

What does APCI mean?

APCI means Application Layer Protocol Control Information and identifies the application service.

What is GroupValueWrite?

It is a common KNX application service used to send a new value to a group address.

What is GroupValueRead?

It requests the current value associated with a KNX group object.

What is a KNX Datapoint Type?

A DPT defines how application data should be interpreted, including data width, scaling and semantic meaning.

Can the decoder determine temperature from raw bytes?

Only when the correct Datapoint Type is known. Raw telegram bytes alone do not prove that a value represents temperature.

How is the KNX checksum checked?

XORing all telegram bytes including the checksum should produce 0xFF for a valid standard KNX TP frame.

Decode KNX TP Telegram Bytes

Inspect KNX control fields, individual and group addresses, routing counters, TPCI/APCI services, application bytes and checksum validity directly from raw hexadecimal telegrams.