Serial Interface Rs485; Overview - Danfoss VLT Refrigeration Drive FC 103 Design Manual

System Integration

3.9 Serial Interface RS485

3.9.1 Overview

RS485 is a 2-wire bus interface compatible with multi-drop
network topology. 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 1 network segment.
Repeaters divide network segments, see Illustration 3.41.
NOTICE
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
biassed termination resistor network. Always use screened
twisted pair (STP) cable for bus cabling, and follow good
common installation practice.
Illustration 3.41 RS485 Bus Interface
MG16G202
Design Guide
Low-impedance ground connection of the screen at every
node is important, including at high frequencies. Thus,
connect a large surface of the screen to ground, for
example, with a cable clamp or a conductive cable gland.
It may be necessary to apply potential-equalising cables to
maintain the same ground potential throughout the
network, particularly in installations with long cables.
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
Impedance [Ω]
Cable length
[m]
Table 3.22 Cable Specifications
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Screened twisted pair (STP)
120
Maximum 1200 (including drop lines)
Maximum 500 station-to-station
3 3
71