X20 system modules • X20 electronics module communication • X20CS1012
1480
X20 system User's Manual 3.10
4.18.3.7.5 Calculating the resistive bus length
The resistive cable length must be calculated in order to ensure a sufficient power supply of 12 V on the M-Bus.
What matters most here is the longest segment between the master and slave.
The resistive bus length is calculated using the following formula without taking an increased bus current caused
by a defective receiver into account:
L
res
=
V - (n * 0.0015 + 0.02) * 6 - 12.6
I/O
(n * 0.0015 + 0.02) * R
L
* 1000
L
res
... Resistive bus length [m]
n ...
Number of slaves (all at end of line)
R
L
... Line resistance (loop resistance [Ω/km])
V
I/O
... I/O supply voltage [V]
Examples for calculating the maximum resistive bus length:
No.
Example
Maximum resistive bus length
1
•
64 slaves (all at end of line)
•
19.2 V I/O supply voltage
•
0.5 mm² wire cross-section
675 m
2
•
64 slaves (all at end of line)
•
28.8 V I/O supply voltage
•
1.5 mm² wire cross-section
5340 m
4.18.3.7.6 Accounting for the capacitive bus length
The total distance of the network is referred to as the capacitive bus length (the sum of all segment lengths). The
capacitive bus length depends on two factors:
•
Distributed capacitance of cable
•
Transfer rate
Distributed capacitance of cable
A lower distributed capacitance on a cable means a higher capacitive bus length.
Transfer rate
A lower transfer rate on an M-Bus system means a higher capacitive bus length.
Example of a cable with a distributed capacitance of 50 nF/km:
Transfer rate
Capacitive bus length
9600 bit/s
1 km
2400 bit/s
4 km
300 bit/s
10 km
4.18.3.7.7 Bus installation
Cables with twisted pair wires and a cross-section of 0.5 mm² to 1.5 mm² are normally used for bus installation
(according to standard: J-Y(ST)Y nx2x0.8). The shield on shielded cables only has to be grounded to the module
on one side. On the slaves, the shielding must be high resistance for DC and low frequency signals.
4.18.3.7.8 Repeater
Repeaters can be used to further expand the M-Bus network.
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