3.8 Serial Interface RS485
3.8.1 Overview
RS485 is a 2-wire bus interface compatible with multi-drop
network topology, that is, 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.
Repeaters divide network segments, see Figure 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 adjustable frequency
drives or a biased termination resistor network. Always use
shielded twisted pair (STP) cable for bus cabling and follow
good common installation practice.
Low-impedance ground connection of the shield at every
node is important, including at high frequencies. Thus,
connect a large surface of the shield to ground, for
example, with a cable clamp or a conductive cable
connector. It may be necessary to apply potential-
equalizing 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 adjustable frequency drive,
always use shielded motor cable.
Cable Shielded twisted pair (STP)
Impedance [Ω]
120
Cable length [m
(ft)]
Maximum 1200 (4000) (including drop lines)
Maximum 500 (1650) station-to-station
Table 3.24 Cable Specications
Figure 3.41 RS485 Bus Interface
System Integration
VLT
®
AQUA Drive FC 202
80 Danfoss A/S © 09/2014 All rights reserved. MG20N622
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