KNX goes metering

Starting with the first design concepts of the KNX RF communication medium, the KNX Association worked together with CEN TC294 WG5 to streamline 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.

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Aspects that were reviewed included FSK deviation, frequency tolerance and a basic common frame format, up to dedicated service and API provisions in the devices. With this level of common definition 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, whereas normally this form of device is a central unit.

So what does this look like in practice? M-Bus RF metering devices may be spread all over a building, which may encompass multiple apartments or office floors, each equipped with one or more M-Bus as well as KNX RF devices. In such a building, one common KNX TP network is often available or may be installed, and metering data are easily captured through 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, and thanks to the seamless integration of KNX RF with M-bus, this is now possible. Once on KNX, metering data may be readily transported over, say, KNX TP and IP, and made available to an operator or service provider locally or remotely (e.g. through the internet).

In order to ensure correct representation and data integrity, a Metering Data Collector, to be hosted in the single RF-to-wired KNX coupler referred to above, was modelled. 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 then accessible to the building (or site) gateway.

This mechanism provides access to the most important data on energy consumption, including current values, minima, maxima, averages etc. Here advantage can be taken 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.

And in order to ensure strong runtime interworking across application domains, this mapping is richer than described, with part of the M-Bus data additionally provided in the KNX system’s runtime Group Address format, for sharing with other applications and for visualisation purposes. This initiative brings together industry sectors from different backgrounds and with different views of markets and target groups. The challenge now is to the manufacturers and users to convert these concepts into real business.