Integration of a KNX bus into Domat controllers

The KNX bus is most often installed in residential objects like family houses, but also on office buildings for illumination and blinds control, or heating and air-conditioning of individual rooms. If there is a building control system installed on the premises, it may be useful to bring all data to a common SCADA or even to interconnect both systems on the automation level.

One of the devices to complete this task is the IntesisBox IN701KNX1000000 (or older version IBOX-MBS-KNX-100) converter which is able to integrate up to 100 KNX variables and present them in a Modbus table. Extended versions provide 500 and 3000 variables. The KNX engineer shall provide to the BMS engineer a table with variables to be integrated, together with their KNX addresses and data types, e.g.:

  • temperature room 112, 0/1/0, float 16bit,
  • switch room 101, 0/0/1, 1bit,
  • light room 107, 0/0/2, 1bit, etc.

When choosing between Modbus TCP and Modbus RTU, the required response time, PLC capabilities, and number of variables. Modbus TCP offers higher data throughput in general, and therefore is more suitable for larger projects. 

A Modbus RTU topology is usually used at smaller sites, or at PLCs which do not provide Modbus TCP channels.

The IntesisBox configuration using the LinkBoxMB software (supplied free of charge together with the converter) is fairly easy and consists of converter parameters settings (IP address and other network parameters, if Modbus TCP is used, RS232 or RS485, if serial interface is used), converter KNX address, and several other parameters, and of course of the Modbus table definition with addresses and variables properties at the KNX bus, which correspond to the Modbus registers.

In the variables table, for each Modbus register there is the KNX value type (EIS), variable address,  KNX read / write rights, variable format, Modbus register number, and for binary values  access over Modbus bit-oriented functions, and Modbus read / write rights.
After the configuration (or „project“) has been uploaded to the converter, establishing of communication is a matter of several seconds. It is recommended to test the communication using a general Modbus client (e.g.  Domat ModComTool) before configuring the devices in Merbon IDE.

The ModComTool communication for the configuration shown above looks like this:

Note that the temperature in Register 2 shows 22 °C multiplied by 10 to achieve resolution of one decimal position. (Modbus only transfers integer numbers). The input linear transform („Kx + Q“) in Merbon IDE must be set accordingly.

The KNX bus integration makes connections between primary controls (heating, ventilation, energy production) and secondary control (zones, rooms) easy, and helps to bring the KNX variables to SCADA where the values may be further processed using trending, alarming, logging and the like.