Page 5 - Advanced Course

KNX ADVANCED COURSE

The previously described method of operation can also of course be implemented using
switch actuators or controllers. To do so, a sensor or controller which is able to send a
telegram cyclically must be configured so that it continually retriggers a time switch or
staircase lighting function in the output device without the output device changing its
status. If information is lost (e.g. the power supply of a primary line fails), the output
device changes to the status that signals the alarm state. Using this method, it would be
possible to check from a central location whether all the line segments of a KNX system
are supplied with voltage. You would simply need to install a bus device in each segment
which sends a cyclical telegram and a device in a central location which evaluates this
information.

Figure 3: Sensor repeats cyclically

In event-controlled, distributed systems, the majority of security functions are based on
the repetition of telegrams, which the input device i.e. the sensor repeats cyclically.

2.2 Priority of a Telegram

A further step in enabling telegrams to be sent as quickly as possible is the setting of
priorities at the communication object. Normally, the appropriate priority levels are set by
the manufacturer. If however an application should require the priority levels to be
modified, they can be converted at the communication object in the ETS program.
The following priority levels are available:
Low operational priority
High operational priority
Alarm
System (used by ETS when downloading)

The following diagram indicates a weather station in which object 1 (Output Safety 2) has
been set to ‘Alarm’ priority. This setting firstly causes more logical “0s” to be sent in the
check byte of the telegram and the telegram immediately takes precedence over a device
with ‘Auto’ or ‘Normal’ priority. It should however be noted that you should not select the
same priority level for a large number of telegrams which could be transmitted at the
same time.

Home and Building Management Systems KNX Association
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