Baxi LUNA SAT RCZ G1-MB Installation And Operating Manual page 50

12.1.1 M-Bus System Design
Before installing an M-Bus system, a number of factors should be considered:
• Number and type of M-Bus meters used
• Layout of the devices in the system
• Number and type of central units and converters to use
• Appropriate assembly position of the central unit, signal converter and any signal repeaters. (They are usually installed
in the electrical panel of the boiler room).
• Distances between the various system devices
• Bus cable: type, length and section
• Route of bus communication cable
• Data transmission frequency
• Management of M-Bus system
The main aim of the design of an M-bus system is to create the documentation allowing the M-bus system and network
to be properly managed and serviced.
Procedure
1. Prepare a diagram of the bus system: enter all the M-Bus devices used with relative distances between them.
2. Choose the route of the bus cable: choose the shortest route for the bus cable in order to reduce the distance. The
star connection typology is best in case of problems with the network as it is quicker and easier to disconnect the
bus. Even though the opposite may seem true, the linear typology requires less bus cable. The most popular solution
is a combination of the two types, i.e.: the tree typology.
3. Determine the number of centralisation components to use: central unit, signal converter, any signal repeaters with
relative locations. The number of M-Bus devices to centralise determines the quantity of central units and converters
to use.
4. Check the distances of the bus: two factors should be considered:
• Minimum voltage of the bus to the M-Bus devices (slaves)
• Maximum transmission frequency
Sizing
• The total length of the cable, the connected M-bus devices and the relative line protections produce capacitive char-
ges in the M-bus segment that reduce the data transmission speed.
• Maximum transmission speed can be determined using the following reference table:
Total capacitive charge of the M-bus segment
Up to 382 nF
Up to 1528 nF
Up to 12222 nF
• The lowest speed of communication calculated among the various segments determines the maximum speed of
transmission that can be used in the system. If a higher than permitted transmission speed were used, the system
would not be able to find some or all of the connected devices.
Each M-bus segment must deliver the minimum voltage to the M-Bus devices; if not, the devices will not be detected by
the central unit.
Example of distances
The following table shows examples of applications that have been especially studied to calculate the maximum distan-
ces of the cable whilst guaranteeing the minimum voltage on the bus and the transmission frequency.
INSTALLATION AND OPERATING MANUAL
Maximum transmission speed
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7215037.01 - en