User Manual
15-3
Siemens Switzerland Ltd www.siemens.com/gamma 2019
Building Technologies Division
Technical Information and Application Examples
Technical Information
System overview
Topology
KNX is hierarchically structured and can be adjusted individually to the
size of the installation. The smallest part in a KNX installation is a line.
Each line, also the main line, includes up to 64 bus devices. Via the line
coupler (LC) a maximum of 15 lines can be combined to a functional
zone.
Installation system with KNX - one functional zone with 15 lines
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
64
1
2
3
4
........................
12
13
14 15
64 6464
64
64 64
64
64
64 64 64
I201_05798
LC Line coupler
Line
LC LC
LC
LC LC
LC LC LC
LC LC LC
LC
Via the backbone coupler (BbC) the 15 functional zones can be
combined once again. Thus, more than 14.400 KNX devices can be
controlled and such a KNX installation can be further expanded via
repeaters.
Installation system with KNX - 15 functional zones combined
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
64 64 6464
64
64 64
64
64
64 64 64
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
2
3
4
5
6
7
8
9
10
11
12
13
14
15
I201_05796a
BbC
BbC
BbC
BbC
BbC
BbC
BbC
LC LC LC
LC
LC LC LC
LC
LC LC LC
LC
Functional zone 1
LC
BbC
1.... Bus devices
Line coupler
Backbone coupler
To other systems
If more than 64 devices are necessary in one line, up to 4 line seg-
ments can be combined via a line amplifier. This line amplifier can also
be used to extend the line length beyond the maximum line length.
One line segment needs one KNX power supply.
For each line segment there are the same rules about the maximum
amount of devices to connect and the distances to each other as well
as the length of the lines. In one line a maximum amount of further 3
line couplers is allowed, leading to a maximum amount of 4 times 64,
i.e. 256, bus devices allowed to be installed in one line. At the main
line and the functional zone there are no line amplifiers allowed. With
each line having its own power supply, each line needs to galvanically
isolated using line/backbone couplers. This guarantees that a failure of
a line does not impact the remainder of the system.
Another advantage of separating a system in lines and functional
zones is to limit the transmission of such data to the functions within
one line. The line and backbone couplers transfer only those telegrams
which are relevant for these lines or zones. This also enables a parallel
communication in several lines and functional zones at the same time.
Due to the hierarchical organization of a KNX system, a clear commis-
sioning, diagnose and maintenance is possible any time. In case the
disciplines shall be structured independently, the topology or the sys-
tem design with lines and functional zones provides suitable solutions.
The system can be structured like this: line structure, star structure,
tree structure and a mixed structure out of these three designs. Within
a line there are the following wire installation rules to note:
• Maximum length of wire in one line: 1000 m
• Maximum distance between two participants: 700 m
• Maximum distance between participant and power supply: 350 m
Transmission Technology
The individual bus participants exchange information via telegrams.
The telegram contains for example switching commands or status
messages. As the bus wire is symmetrically structured and the wiring is
installed floating potential free to Earth ground, a potential diffe-
rence between the two wire cores does not cause disturbances with
reference to the earth potential. Regarding transmission speed, pulse
generation and pulse reception the transmission technology is de-
signed that no termination resistor is necessary and any wiring topo-
logy is possible. The transmission rate of KNX TP is 9600 bit/s, which
are about 40 to 50 telegrams per second.
The bus access of each KNX product secures a well ordered information
exchange. This is guaranteed by a serial and asynchronous transmis-
sion on the bus line. For increasing the reliability and targeting highest
transmission rate, KNX applies the CSMA/CA- bus access procedures
(Carrier Sense Multiple Access with Collision Avoidance). With the help
of this procedure no telegram gets lost when several KNX users are
sending telegrams and the higher prioritized telegram gets through
the line first.
All KNX participants are always listening to the bus. The participant
decides on its own when the telegram is sent to the bus as long as the
bus is not busy with transmitting information. The transmission of KNX
is event-driven, that means, the telegrams are only sent to the bus
when the event actually happens and the transmission of information
is necessary.
1515
39_VS_2017_Gamma HQ_EN_Chapter_15_V1.indd 3 26.07.2018 15:24:47