Table 8: Modbus RTU rules
Category Rules and Maximums
General
One direct Modbus RTU (RS-232) connection to either RS232C port is supported;
connection requires an RS-232/RS-485 converter to the RS232C serial port
One direct Modbus RTU (RS-485) connection to either FC Bus port is supported
Only daisy-chained devices
Number of Devices
RS232C port supports one Modbus RTU (RS-232) device.
RS232C port with RS-232/RS-485 converter supports 32 Modbus (RS-485) devices or
more, depending on the converter used
Note: The typical maximum Modbus device count between repeaters is 32.
Supported Modbus RTU serial device count may be limited by manufacturer,
cable, and communication speed. Check with the manufacturer for more
information.
Cable Length
RS-232 cable length can be up to 15 m (49.2 ft)
RS-485 cable length can be up to 1,520 m (4,987 ft)
Cable
RS-232 stranded cable, 3-9 conductors, serial data grade, 20–24 AWG
RS-485 stranded cable, 0.6 mm (22 AWG) 3-wire twisted, shielded cable is
acceptable. Standard wiring qualified for use with the N2 Bus or FC Bus is
acceptable
Stranded 0.6 (22 AWG) 4-wire (two twisted pairs) shielded is acceptable
Terminations
RS-232: No termination
RS-485: End-of-line (EOL) termination must be set to On (or an EOL terminator
installed) on the two devices located at either end of each bus segment on an
RS-485 bus. The EOL switches must be set to Off (or EOL termination disabled) for
all other devices on the bus segment on an RS-485 bus.
Table 9: 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.
NAE55/NIE55 Installation Guide 17
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