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In big networks using secondary addressing worst case load must be considered as 250 slaves of 1 UL (Unit Load) each can
draw 5.4 A, which thin cables will not be able to withstand.
Please note that resistance can be stated in two different ways in cable specifications; i.e. as cable resistance or as loop
resistance.
Loop resistance is the total resistance measured through the two conductors. Cable resistance is the resistance through one
conductor. Therefore, loop resistance is always twice the cable resistance.
Measurement of loop resistance
Measurement of cable resistance
4.3.2
Cable topology
An M-Bus network normally uses bus or star topology, or a combination of both.
The advantage of bus topology is shorter wires. The disadvantage is that a cable interrupt will mean that all the following
meters can no longer be read.
The advantage of star topology is the fact that all the other meters can still be read after a cable interrupt. The disadvantage
is a large consumption of cable with a high leading load, which reduces the maximum cable length and makes a reduction
of communication speed necessary respectively.
Bus topology offers two solutions, one looping the cable through each meter. This solution presupposes that there is room
for two cables and that the connection terminals are prepared for the connection of two sets of cables. All connections are
thereby typically made inside the meter.
Using bus topology with each individual meter connected to the bus, a number of connections must be established on the
bus itself.
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55121391_B1_GB_12.2013
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