16
Modbus RS-485 Installation and Set-up
RS-485 is a two-wire bus interface compatible with multi-
drop network topology, i.e. nodes can be connected as a
bus, or via drop cables from a common trunk line. A total
of 32 nodes can be connected to one network segment.
Network segments are divided up by repeaters. Please
note that each repeater functions as a node within the
segment in which it is installed. Each node connected
within a given network must have a unique node address,
across all segments. Terminate each segment at both
ends, using either the termination switch (S801) of the
frequency converters or a biased termination resistor
network. Always use screened twisted pair (STP) cable
for bus cabling, and always follow good common
installation practice. Low-impedance ground connection
of the screen at every node is very important, including at
high frequencies. This can be achieved by connecting a
large surface of the screen to ground, for example by
means of a cable clamp or a conductive cable gland. It
may be necessary to apply potential equalizing cables to
maintain the same ground potential throughout the
network, particularly in installations where there are long
lengths of cable. To prevent impedance mismatch,
always use the same type of cable throughout the entire
network. When connecting a motor to the frequency
converter, always use screened motor cable.
Cable: Screened twisted pair (STP)
Impedance: 120 Ohm
Cable length: Max. 1200 m (including drop lines)
Max. 500 m station-to-station
Connect the frequency converter to the RS-485 network
as follows (see also diagram):
1. Connect signal wires to terminal 68 (P+) and
terminal 69 (N-) on the main control board of the
frequency converter.
2. Connect the cable screen to the cable clamps.
Screened, twisted-pair cables are recommended
in order to reduce noise between conductors.
The following EMC precautions are recommended
in order to achieve interference-free operation of the RS-
485 network.
1. Relevant national and local regulations, for
example regarding protective earth connection,
must be observed. The RS-485 communication
cable must be kept away from motor and brake
resistor cables to avoid coupling of high
frequency noise from one cable to another.
2. Normally a distance of 200 mm (8 inches) is
sufficient, but keeping the greatest possible
distance between the cables is generally
recommended, especially where cables run in
parallel over long distances.
3. When crossing is unavoidable, the RS-485
cable must cross motor and brake resistor
cables at an angle of 90 degrees.
See the Dan Foss Design Guide for more detailed
Modbus information
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