KNX data request rate

What controls the rate of KNX data requests from the driver?

The Inter Message Delay (see Knx Device/Comms Connections/Tunnel Conn) setting in the Tunnel Connection exists to ‘pace’ the out-going data requests from the driver. This can be used to reduce the rate of data requests in cases where a KNXnet/IP Interface device cannot cope with the traffic volume, caused possibly by its implementation settings or by its operating speed. The Inter Message Delay default setting of minimum 15ms has proved successful with Siemens interface devices. Any problems arising from this being set too small would manifest as Control Points intermittently having a Read Fault: Timed out waiting for L_Data_con fault condition.

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Each out-going data request actually involves 6 packets travelling between the KNXnet/IP driver and the KNXnet/IP Interface as follows:

  • Request from KNXnet/IP driver to KNXnet/IP Interface
  • Acknowledgement from KNXnet/IP Interface to KNXnet/IP driver
  • Confirmation from KNXnet/IP Interface to KNXnet/IP driver
  • Acknowledgement from KNXnet/IP driver to KNXnet/IP Interface
  • Reply from KNXnet/IP Interface to KNXnet/IP driver
  • Acknowledgement from KNXnet/IP driver to KNXnet/IP Interface

This appears as several flashes on the KNXnet/IP Interface’s LEDs.

NOTE: There is an important difference in the implementation of Inter Message Delay between the KNXnet/IP driver and the EIBnet/IP driver. In the EIBnet/IP driver the Inter Message Delay was applied between all out-going messages, including ‘Acknowledgements’ and all connection control messages. In the KNXnet/IP driver the Inter Message Delay is only applied between data request messages.