Technical data
64 Theoretical and general applications www.westermo.com
same type of transceiver and be able to send network variables. Nodes also need to
understand the contents of the network variables. For example, nodes must know
whether a temperature is stated in Fahrenheit or Celsius or whether a flow is stated in
litre/second or millilitres/second. Thus standards for how the content of these data
packets should be interpreted are necessary. Within L
ONW
ORKS
®
standardisation is
managed by an organisation called the L
ONM
ARK
®
Association. This is an independent
association consisting of manufacturers of L
ON
WORKS
®
-nodes, system integrators and
end-users. They have compiled a list of standardised types of network variables. These
types are designated SNVT (pronounced snivit), which stands for “Standard Network
Variable Types”. These types contain information about the device, resolution and
which values the type can take. For example, when the type SNVT_speed is used, all
L
ONW
ORKS
®
-nodes know that the unit is metre/seconds, the resolution is 0.1 metre/
seconds and it can take values between 0 to 6553.5 metre/seconds.
The most employed transceiver is the FTT-10A free topology. It communicates at a
rate of 78 kbit/s over a twisted pair cable. Free topology means that it can be used in
star networks, ring networks, bus networks or combinations of these. Echelon
®
also
has a free topology transceiver called LPT-10 LinkPower. This is compatible on
a signal level with the FTT-10A and can be used together with this. What is special
about LPT-10 is that it is “true 2-wire” in the meaning that the wire transfers both data
and power. The advantage of being able to freely mix topologies means that these
transceivers are extremely useful in today’s control networks where you need to be
able to add new devices easily. Another advantage of these transceivers is that they
have a polarity insensitive connection, which aids installation and eliminates the risk of
incorrect connections. Other transceivers from Echelon
®
include the 1250 kbit/s twist-
ed pair wire transceiver for bus topology and a transceiver for electrical network com-
munication. The ability to jump between two frequency bands, advanced signal treat-
ment and error correction enables the electrical network transceiver to easily handle
disturbances from for example motors, dimmers, PC and televisions.
The PLT-22 transceiver can be configured to either communicate over the power net-
work on the public frequency band Cenelec-C or the Cenelec-A frequency band
which is reserved for power companies. The C-band is usually used for applications
within intelligent housing and other commercial applications while the A-band is often
used in connection with electricity meter reading. There are also third party trans-
ceivers for fibre optic, radio and IR communication. It is common for different media to
be combined in a L
ONWORKS
®
network. Echelon
®
offers routers that can forward
B A C K