M-Bus protocol rules
Note:
Unit load is a defined standby current. A device is permitted a current drain of one
unit load by default but may consume more if it is shown at the device (by an integer) and in
documentation. Use M-Bus Repeaters to increase the length and the number of unit loads
permissible.
Table 7: Rules for M-Bus protocol
Category
Rules/maximums allowed
General
Supports one direct M-Bus serial (RS-232) connection to the RS232C port.
Connection requires an RS232-to-M-Bus Level Converter on the RS232C
port.
FC port does not support M-Bus integration.
No restrictions in topology, but Bus topology is strongly recommended.
Number of devices
Depends on level converter (logical maximum is 250 devices).
Length depends on cable resistance, capacitance, number of devices,
position of devices, and configured communication speed. Example
scenarios to help with calculation:
Baud
rate
Maximum number
of unit loads
Maximum
distance between
converter and
devices
Maximum
distance for entire
Bus
64
3,000 m (9,842 ft)
5,000 m (16,404 ft)
(2 x 1.0 mm (18
AWG), shield
recommended,
resistance < 90
Ohms)
64
1,000 m (3,281 ft)
4,000 m (13,123 ft)
(2 x 1.0 mm (18
AWG), shielded,
resistance < 90
Ohms)
2,400
250
350 m (1,148 ft)
4,000 m (13,123 ft)
(2 x 0.8 mm (20
AWG), shielded,
resistance < 30
Ohms)
Line length and type
9,600
64
350 m (1,148 ft)
4,000 m (13,123 ft)
(2 x 0.8 mm (20
AWG), shielded,
resistance < 30
Ohms)
NCE25 Installation Instructions
12