Baxi LUNA SAT RCZ-MB Installation And Operating Manual page 49

Operating Principles
• The operating principle is based on the "Single Master Slave", that is, just one Master M-bus can be used.
• Data transmission is always determined by the central unit. The central unit queries the various devices on the bus
(Slave) and not vice-versa.
• The transmission method is asynchronous, halfduplex.
• All bus layout typologies are acceptable except the loop layout!
Bus cable
• The cable to use must be twisted with 2 conductors (unshielded)
• The polarity of the device connections can be interchangeable but the polarity should be observed in the connections
to the various networked devices (slaves).
• The M-bus does not require an end-of-line termination device.
Transmission frequency
• The M-bus supports various communication speeds: 300, 2400 and 9600 Baud. More than one speed can be used
at the same time.
• The maximum transmission frequency depends on the type of M-Bus devices, on the distances, on the number of
devices and on the type of cable used in the M-Bus system.
• To calculate the transmission frequency, refer to the instructions supplied with the accessories.
Distances
• The M-bus can reach considerable communication distances with more than 10 Km of cable (N.B.: with just one de-
vice and with a 1.5 mm
2
• The maximum distance still depends on the number of M-Bus devices, on the transmission frequency, on the route of
the bus cable and on the type of cable used.
• The M-bus network can be extended using signal repeaters.
• To calculate the distances that can be reached, read the instructions provided with the accessories.
Addressing
The M-bus uses two address typologies to detect the devices in the field:
Primary address and secondary address. The use of the two addressing types can be combined within the same system.
Primary Address
In an M-bus system, a maximum of 250 primary addresses can be assigned (hexadecimal logic). The primary address
is normally assigned during start-up in order to logically sort the centralised devices. The default primary address of the
devices is "0". With more than 250 devices connected, secondary addressing is required.
Secondary address
The secondary address comprises 8 Bytes and allows any number to be assigned. By default, the secondary address
of the devices is equal to the factory number. This prevents conflicts during searching on the bus. Using the secondary
address, the devices can be received by the central unit without any specific address having to be assigned. If required,
the detected devices can be logically sorted at a later stage.
Search logic
The central unit searches the connected devices on the bus via the primary address, the secondary address or the
primary and secondary address.
After starting the search, the central unit (MASTER) first searches the devices on the bus (SLAVE) saving them to a volatile
memory; after detecting all the connected devices, the central unit sorts their assigned addresses in increasing order: 1,
2, 3, 4...
Searching with primary address
Searching and sorting by primary address is faster as the central unit searches and sorts the devices with addresses
having no more than 3 digits (1...250)
Searching with secondary address
Searching and sorting by secondary address is slower as the central unit searches and sorts the devices with addresses
having 8 digits (00000000...99999999)
INSTALLATION AND OPERATING MANUAL
12.1 GENERAL DATA OF THE M-BUS SYSTEM
bus cable). All the same, the bus cable should not exceed a distance of 4 Km.
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