After a reset, it takes up to 5 minutes for all connected room controllers to restart. This
is designed to avoid overloading the mains power supply at startup. At the same time, it
reduces the load on the KNX network, as not all controllers transmit data at the same
The delay is determined by the device address of the controller.
In a large KNX system, bus load can be a problem especially with central commands
which cause the controllers to send state information simultaneously. This can even
result in the loss of data telegrams.
The management station is normally connected to the KNX bus via the area line. This
line thus requires special attention on bus load, as it can become the system's
bottleneck: The bus load acceptable on individual lines results in overload on the main
lines and area line.
To avoid a KNX communications overload, note the following rules and precautions:
• Define a rational topology, divided into lines and areas.
• Avoid cross-line functions: place sensor and actuators on the same line if possible.
• Load filter tables for the line and area couplers so that local information does not
burden the entire system.
• Meaningful integration in management station or visualization: Integrate only
information that is really required.
• For state messages, automatic transmission is not necessary if the display device
can actively read the values. The flags must be set accordingly in the ETS.
• Do not define too short an interval between heartbeats.
• Divide central commands which affect a large number of controllers over several
group addresses, and stagger transmission.
• Each group address must have a recipient to acknowledge the telegram, otherwise
unwanted repeat-telegrams are generated.
• Further notes on reducing bus load, see pages 23 and 83.
RXB (KNX) Applications library
CLC and RAD description of functions for CC-02 CM110384en_03
28 Feb 2009