Johnson Controls Metasys NAE55, Installation Manual, Page: 14 / 20

Table 15: M-Bus protocol rules (bus topology)
Category Rules
General One direct M-Bus serial (RS-232) connection to either RS232C port is supported; connection requires an
RS232-to-M-Bus Level Converter on the RS232C serial port.
One direct M-Bus (RS-485) connection to either FC Bus port is supported; connection requires an RS485-
to-M-Bus Level Converter on the FC Bus port.
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
1
1
,
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 16: 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)
NAE55 Installation Guide 14