Table 6: Modbus RTU rules
Category
Rules and Maximums
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 7: M-Bus protocol rules (bus topology)
Category
Rules
General
One direct M-Bus serial (RS-232) connection to the RS232C B port is
supported.
Connection requires an RS232-to-M-Bus Level Converter on the RS232C port.
FC-A and FC-B support one M-Bus RS-485 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
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 <
90Ohms)
Line Length and
Type
2400
250
350 m (1,148 ft)
4,000 m (13,123 ft)
(2 x 0.8 mm [20
AWG], shielded,
resistance <
30Ohms)
NAE55/NIE55 Installation Guide
24
