KNX Metering Specification
Smart homes and buildings employing KNX as their control network may be realised with any suitable combination of the communication media KNX Twisted Pair (the ubiquitous green cable!), Powerline Carrier, Internet-based communication (IP) or Radio Frequency (RF). Now it is precisely the KNX RF medium that supplies the link to metering applications.
Indeed, starting with the first design concepts of the KNX RF communication medium, KNX Association worked together with CEN TC 294 WG5 and streamlined the parameters of the KNX RF Physical Layer (868 MHz-standard CEPT/ERC 70-03) and the Data Link Layer (based on the FT3-protocol IEC870-5-2) with the M-Bus specifications. This concerns aspects as FSK deviation, frequency tolerance as well as a basic common frame format, up to dedicated provisions (services, API) in the devices.
“With this level of common definitions and integration, it is possible to have a KNX product that receives both M-Bus telegrams as well as KNX telegrams with only a single receiver. Normally this device is a kind of a central unit,” says Steven De Bruyne, Systems Manager at KNX Association, and secretary of the KNX Task Force “Metering”. He adds: Mr. Pahl (Qundis) has done a great job in setting out common solutions with the metering people and the M-bus world. The M-Bus community has also actively contributed to bringing concrete technical proposals to paper. The approach is a very pragmatic one, so the step towards implementation is small, and products have already been announced.”
So what does this look like in practice? The M-Bus RF metering devices may be spread all over the building, which may encompass multiple apartments or office floors, each equipped with several M-Bus as well as KNX RF devices. In such buildings, one common KNX TP network is often available or may be installed; metering data is easily captured through a minimum number of couplers in the installation. Clearly, the installation and configuration costs can be lowered if this gateway provides access to both the metering and KNX data. Thanks to the seamless integration of KNX RF with M-bus, this becomes a piece of cake.
Once on KNX, metering data may be readily transported over, say, KNX TP and IP from there, and made available to an operator or service provider locally or remotely (e.g. through Internet), as shown in the picture.
Steven De Bruyne further points out one particular challenge: “Among many others, KNX provides applications for heating and cooling. This makes process- and sensor data available to the customer. The integration with M-Bus completes this picture with consumption data. Care has to be taken concerning correct representation and data integrity, while maximising the advantage for all partners involved!”
To achieve this, the Task Force has modelled a Metering Data Collector, to be hosted in the single RF-to-wired KNX coupler referred to above. To begin with, the Metering Data Collector maps a limited and well-defined subset of M-Bus metering data to a structured, KNX compliant data interface (namely: properties of KNX Interface Objects), where they are now accessible to the building (or site) gateway.
This mechanism provides access to the most important data on energy consumption: current values, minimum, maximum, average… Here, we can take advantage of the “array” aspect of KNX properties, to support such complementary metering data (corresponding to multiple storage numbers on M-bus). Flexibility for metering applications is in no way curtailed, as raw metering formats may also be transported.
But, true to the KNX tradition of strong run-time interworking across application domains, we wanted this mapping to be richer than what we just described. This is why part of the M-Bus data is additionally provided in the KNX system’s runtime Group Address format, for sharing with other applications and for visualisation purposes – based on a careful selection, defined in co-operation with M-Bus manufacturers.
The challenge is now up to the manufacturers and users to convert these concepts into real business.