Two Parts, One Division of Labour
Every KNX device functionally splits into two halves. The bus coupling unit handles everything to do with the bus: it hangs on the two-wire line, draws its operating voltage from it, receives and sends telegrams and holds the individual address. The application module does the actual work, meaning switching, dimming, measuring or picking up a key press.
This division is the reason a manufacturer does not have to develop a complete new device for every function. The bus part is the same everywhere, only the attachment changes.
The two are connected through a standardised 10-pole interface, the application interface or PEI. It is the reason attachments of various designs can be plugged onto the same flush-mounted bus coupling unit.
The Address Sits in the Coupler, Not in the Attachment
This is the point that matters in practice and that introductory texts like to leave out: **the individual address is written into the bus coupling unit.** The attachment does not carry it.
Two things follow. A faulty or unwanted button sensor can be pulled off and replaced by another without anything being readdressed. And conversely: anyone replacing the bus coupling unit replaces the identity of the device and must give it the address again, even if the attachment stayed the same.
The same applies when reprogramming. The programming button and its LED belong to the bus coupling unit, and with an attachment fitted they are often out of reach. That is not a defect but a matter of design.
Where It Is Still Encountered as a Separate Device Today
The separate bus coupling unit with a pluggable application module was the usual design for a long time and still is for flush-mounted operating elements. You buy a box with a coupler and choose the attachment separately, often from the switch range being installed in the house anyway.
With DIN rail devices in the distribution board it is permanently built in. A switching actuator is one device and not two; the bus terminal sits at the top, the programming button on the front. Anyone buying an actuator today does not think about the bus coupling unit, and that is fine.
What Applies When Mixing Manufacturers
The 10-pole interface is standardised; the combination of coupler and attachment is not in every case. Every bus coupling unit carries its own manufacturer identification, and the product database of the attachment expects a matching counterpart.
For planning this means: order coupler and attachment from one manufacturer together, and do not assume arbitrary parts can be combined just because the connector fits. A connector that fits is not yet a release in the ETS.
Key Facts
- Two parts: bus coupling unit on the bus, application module for the function
- Connected via the standardised 10-pole application interface (PEI)
- The individual address sits in the coupler, not in the attachment
- Swapping the attachment costs no new address, swapping the coupler does
- In DIN rail devices it is built in and not a separate topic