Table 9: M-Bus protocol rules (bus topology)
Category Rules
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
11,2
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)
2400
250 350 m (1,148 ft)
4,000 m (13,123 ft)
(2 x 0.8 mm [20 AWG],
shielded, resistance < 30
Ohms)
64 350 m (1,148 ft)
4,000 m (13,123 ft)
(2 x 0.8 mm [20 AWG],
shielded, resistance < 30
Ohms)
9,600
250 350 m (1,148 ft)
1,000 m (3,281 ft)
(2 x 0.8 mm [20 AWG],
shielded, resistance < 30
Ohms)
38,400 64 350 m (1,148 ft)
1,000 m (3,281 ft)
(2 x 0.8 mm [20 AWG],
shielded, resistance < 30
Ohms)
Line Length and Type
Cable Twisted pair cable (shielding optional)
Termination No termination
1 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 (as an integer) and in documentation.
2 Use M-Bus Repeaters to increase the length and the number of unit loads permissible.
2
Table 10: KNX protocol rules
Category Rules and Maximums
General No restrictions in topology
Number of Devices Depends on chosen topology and cable type.
Line Length and Type
Twisted pair cable recommended; length depends on cable resistance, capacitance,
number devices, position of devices, and communication speed.
Cable
Copper, solid and stranded wires with outer sheath, one- or two-twisted pair; 0.8 to
1.0 mm (20 to 18 AWG)
Screen is required and must cover the entire diameter.
Drain wire: Diameter minimum 0.5 mm (26 AWG)
Termination No termination
Manufacturer's
Quality Management
System
At least ISO 9002
NAE55/NIE55 Installation Guide18
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