Relay MR004C - M-Bus Network Planning Considerations; Shielding and Grounding

User manual MR004DL/FA 07.03.02, Version 1.2 7
GmbH, Stettiner Str. 38, D-33106 Paderborn
Planning M-Bus networks:
Two effects must be taken into consideration before projecting M-Bus networks:
1. The signals must not be distorted to much by the influence of the capacitance of
the cable. The capacitance of the network depends on the length of the network,
this means the total sum of cable length. Lower baudrates allow a higher bus
capacity.
2. Each meter must be powered from the M-Bus with a minimum voltage of 24V.
The voltage drop on the bus line is caused by the transmit current of the
communicating slave of up to 20mA, the supply current of the slaves in the
respective bus segment, the internal resistance of the M-Bus master, the
resistance of the cable and the connection resistances of the joints. The distance
between a slave and the master is inverse dependent of the number of slaves in
this segment and of the cable diameter.
The maximum allowed resistance of the cable R
L
between the MR004DL/FA can be
estimated with the following formulas (N = no. of unit load, each 1.5mA):
N slaves at end of cable: R
L
= (9467 - 15 x n) / (n + 14) ΩΩ
N slaves equally distributed: R
L
= (18933 - 30 x n) / (n + 28) ΩΩ
The calculation for a standard phone cable JYSTY nx2x0.8 (75Ω/km, 150nF/km) presents
the following values:
Baudrate 9600 Baud 2400 Baud 300 Baud
Max. total cable length (@150nF/km): 1 km 4 km 12 km
Max. distance to the meter (@75Ω/km)
No. of unit loads in the
respective segment
Equally distributed slaves at end of cable
1 8,4 km 8,4 km
10 6,5 km 5,2 km
30 4,1 km 2,7 km
60 2,6 km 1,5 km
There are remote repeaters (our art.no. DR007) available for extending the network
length and the number of connected slaves. Cascading these remote repeaters allows a
nearly unlimited bus extension. The place where a remote repeater is placed must
provide a mains supply.
Shielding
Tests performed by the M-Bus Usergroup showed the result that a shielding of the
M-Bus cable is not necessary. Please avoid a connection between any of the bus lines
to the shielding or protective ground.