KNX Datapoint Types Explained for Integrators
Understand KNX datapoint types and subtypes with a practical DPT table, ETS rules, mapping examples, and troubleshooting guidance for integrators.
A KNX datapoint type tells devices what a value means and how it is encoded. Without the DPT, a byte is only a byte. It could represent 50% brightness, an HVAC mode, a counter, or part of a date.
This is why a gateway point list that says only “1 byte” is incomplete. Integrators need the main type, subtype, unit, valid range, direction, and behaviour.
Main type versus subtype
The main number defines a family of encodings. The subtype defines the engineering meaning.
For example, DPT 9 is a two-octet KNX floating-point family. DPT 9.001 is temperature in °C; DPT 9.002 is temperature difference in K. They use the same basic encoding but are not semantically interchangeable.
Write the full subtype whenever it is known. DPT 1.xxx is less useful than DPT 1.001 Switch.
Common KNX datapoint types
| DPT example | Size | Typical meaning | Practical use |
|---|---|---|---|
| 1.001 | 1 bit | Switch | ON/OFF command or status |
| 1.002 | 1 bit | Boolean | General true/false state |
| 2.001 | 2 bits | Switch control | Value plus control/priority bit |
| 3.007 | 4 bits | Dimming control | Relative brighter/darker with step |
| 5.001 | 1 byte | Scaling 0–100% | Absolute dimming or position |
| 7.001 | 2 bytes | Unsigned count | Pulse/count values where specified |
| 9.001 | 2 bytes | Temperature °C | Room or supply-air temperature |
| 10.001 | 3 bytes | Time of day | Clock synchronization |
| 11.001 | 3 bytes | Date | Calendar date |
| 12.001 | 4 bytes | Unsigned counter | Larger non-negative count |
| 13.001 | 4 bytes | Signed counter | Signed count |
| 14.xxx | 4 bytes | IEEE floating point | Measurements requiring larger range/precision |
| 17.001 | 1 byte | Scene number | Recall a scene |
| 18.001 | 1 byte | Scene control | Recall or store a scene |
| 20.102 | 1 byte | HVAC mode | HVAC operating-mode enumeration |
This is a working table, not a substitute for the official specification. Many main types have numerous subtypes and application-specific rules.
Encoding details that create integration faults
DPT 5.001 percentage
The KNX payload uses an unsigned 8-bit value from 0 to 255 to represent 0–100%. A gateway may expose the engineering value as 0–100, while another API exposes the raw 0–255. If that choice is not documented, a 50% command can become 19.6% or be rejected.
DPT 9 two-byte float
KNX two-octet float is not the same as a standard IEEE half-float. It uses a sign, four-bit exponent, and signed mantissa with 0.01 scaling. Gateways should implement the KNX format directly rather than treating the two bytes as a generic integer.
Scene numbers
User-facing scene numbers and transmitted encoded values may be presented differently by software. Verify the ETS object description and gateway convention instead of assuming “Scene 1” always appears as raw value 1 at every interface.
Enumerations
An HVAC mode is not a percentage. Map the allowed enumerated values to named states and define behaviour for reserved or unsupported values.
One group address, one clear meaning
Objects linked to a group address must agree on the datapoint type and semantics. ETS can help identify mismatches, but a technically compatible size can still be wrong.
Examples of bad links:
- a temperature difference object linked to an absolute room-temperature address;
- a one-byte HVAC mode linked to a one-byte brightness value;
- a switching command and alarm acknowledgement sharing an address because both are one bit;
- Celsius data labelled as Fahrenheit in a dashboard;
- command object used as feedback when the actuator has a dedicated status object.
Name group addresses with the function and direction, for example:
L02 / Meeting 2.14 / Lighting 1 / Switch Command DPT 1.001
L02 / Meeting 2.14 / Lighting 1 / Switch Status DPT 1.001
L02 / Meeting 2.14 / Lighting 1 / Dim Value DPT 5.001
L02 / Meeting 2.14 / Room Temperature / Actual DPT 9.001
The structure makes commissioning and later API mapping much easier.
Datapoint checklist for a gateway point list
| Field | Example |
|---|---|
| Point name | L02 Meeting 2.14 Room Temperature Actual |
| KNX group address | 4/2/17 |
| Full DPT | 9.001 |
| Engineering unit | °C |
| Valid range | Project-defined, e.g. 0–50 °C |
| Direction | KNX to BMS, read only |
| Update mode | On change plus periodic refresh |
| Invalid/offline behaviour | Mark stale after agreed timeout |
| BMS object | AI-02-214-RT |
| Test value | Compare with calibrated/reference reading |
Point direction needs special attention. “Read/write” can create control loops when both systems believe they are master. Define which system commands and which system reports.
Choosing between DPT 9 and DPT 14
Both represent floating-point measurements, but they differ in size, range, and precision. DPT 9 is common for temperatures and many building values. DPT 14 is a four-octet IEEE 754 value and may be used for measurements that need a different range or representation.
Use the subtype defined by the device application and the relevant KNX functional specification. Do not choose a larger format simply because it looks more precise; every linked device must support the same DPT.
Troubleshooting a DPT mismatch
When a displayed value is wrong:
- Read the sending group object in ETS and note its full DPT.
- Capture the telegram in Group Monitor.
- Compare raw and interpreted values.
- Check the receiving object, gateway mapping, scaling, byte order, and unit.
- Send a safe known test value from the authorized side.
- Confirm status feedback returns using the same agreed semantics.
Typical symptoms include a fixed zero, values 2.55 times too large or small, negative numbers appearing as large positive counts, reversed HVAC modes, or temperatures that change wildly.
Do not “fix” a mapping by applying an unexplained multiplier in the visualization. Correct the DPT or document the conversion at the responsible interface.
DPTs and GEO/semantic building data
Datapoint types are also the foundation for reusable building information. AI assistants, analytics platforms, and KNX IoT clients can answer useful questions only when values carry location, function, unit, direction, and state meaning.
An array of raw numbers is not a digital building model. A maintained point list is.
FAQ
Is data length enough to identify a KNX datapoint type?
No. Multiple DPTs share the same length. The subtype defines semantics, units, and permitted values.
Can DPT 1.001 command and status objects share a group address?
They can be compatible in size, but separate command and feedback addresses are often clearer and more reliable for multiple control sources and integrations.
Why does 50% sometimes appear as 128?
DPT 5.001 encodes the 0–100% engineering range across a raw byte of 0–255. Some tools show the interpreted percentage; others expose raw data.
Where is the authoritative DPT list?
KNX Association publishes the current standardized Datapoint Types specification through its support portal.
Official references
- KNX Association, standardized Datapoint Types
- KNX Association, application-program DPT guidance
- KNXmart, group-address design best practices
- KNXmart, KNX data points for dashboards and analytics
The discipline is straightforward: use the full DPT, preserve the unit and direction, keep command and feedback responsibilities clear, and test a known value end to end.