4 Cabling
Typically unshielded twisted pair cable up to approx. 1.5 mm
2
is used. The cabling topology is typically star or bus or a
combination of both. The connection in M-Bus is independent of polarity and no termination resistance at the end of the
cabling is needed.
If a cable type with shield is used, it is important that the two M-Bus conductors are not connected to ground or shield.
No precise indication as to maximum cable length in an M-Bus network can be given as it depends on various parameters.
The two most important parameters to consider when selecting cable for an M-Bus installation are cable resistance
and cable capacity. Generally speaking the resistance limits the number of M-Bus slaves, and the capacity limits the
communication speed.
Furthermore, we recommend keeping a certain distance between M-Bus cables and other cables in order to minimize noise
from high-power electric machinery.
4.1 Special features of M-Bus Master MultiPort 250L
M-Bus Master MultiPort 250L has been designed with the newest cable driver technology, and is, therefore, rather
insensitive to the capacity of the M-Bus network.
Thus, designing an M-Bus network to be used together with M-Bus Master MultiPort 250L the limiting factor as to possible
cable length will primarily be the cable resistance in the network.
4.2 Electrical conditions in an M-Bus network
According to EN 13757-2, the maximum output voltage from an M-Bus Master must not exceed 42 V. The output voltage
from M-Bus Master MultiPort 250L is 41 V.
•
If the voltage measured over terminals 24-25 is 24 V or more at the most distant meter, there is a high degree of
certainty that all meters can be read
•
If the voltage is between 20 and 24 V, it will probably be possible to read all meters
•
If the voltage is between 18 and 20 V, the meter may be read
•
If the voltage is below 18 V it is most likely that the meter cannot be read
There must be no communication on the M-Bus network when the above measurement is made.
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