Page 217 - Basic Course

KNX BASIC COURSE

10.6 Data rates and cyclical processes

The data rate, which can occur on average or in critical cases, should also be calculated
right from the beginning. Time-critical functions requiring a quick reaction (real-time
applications) should if possible not be planned and designed across lines. In this case
valuable tenths of seconds could be lost when passing through line/backbone couplers.
This applies to dimming, especially cyclical dimming. If such local functions are realized
across several couplers, this will affect the linearity of the variation of light intensity.
Moreover, the start and end delay of the function may in some cases be too high.

You should also pay attention to the total data rate for all devices with cyclical sending.
Cyclical transmitters are all alarm sensors (e.g. wind and rain detectors, weather control
units, smoke detectors, window contacts), which should be continuously monitored to
ensure that they are functioning correctly.

The security level of the bus system does not always increase when setting such a device
to “sending” as often as possible, but more by choosing a rate that is adapted to the
potential bus load.

Even more critical are devices and software packages that cyclically poll the status of their
communication partner. The fact is that this will result in at least two bus telegrams per
data point –the polling telegram as well as the corresponding answer.

In addition, normal bus load can be endlessly “increased” by incorrectly configured line
and backbone couplers. The following section will outline an example, taking into account
the aspects mentioned above. It shall however be noted that with this example is only able
to touch upon a small proportion of all possibilities of a flexible and variable system as
KNX!

Further below (11.3), you can find a calculation example for planning or calculating bus
telegram loads.

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