Installation And Set-Up; Introduction; Hardware Setup; Parameter Settings For Modbus Communication - Danfoss VLT AQUA Drive FC 202 Design Manual

RS-485 Installation and Set...
7 RS-485 Installation and Set-up

7.1 Introduction

RS-485 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 one network segment.
Repeaters divide network segments. Note each repeater
function 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
7 7
good common installation practice.
Low-impedance earth connection of the screen at every
node is important, including at high frequencies. Thus,
connect a large surface of the screen to earth, e.g. with a
cable clamp or a conductive cable gland. If necessary,
apply potential-equalizing cables to maintain the same
earth 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
Cable length
Max. 500 m station-to-station
Table 7.1 Motor Cable

7.1.1 Hardware Setup

Use the terminator dip switch on the main control board
of the frequency converter to terminate the RS-485 bus.
Illustration 7.1 Terminator Switch Factory Setting
148
Design Guide
Screened twisted pair (STP)
120 Ω
Max. 1,200 m (including drop lines)
ON
1 2
Danfoss A/S © Rev. 06/2014 All rights reserved.
NOTICE
The factory setting for the dip switch is OFF.
7.1.2 Parameter Settings for Modbus
Communication
The parameters in Table 7.2 apply to the RS-485 interface
(FC-port)
Parameter Function
8-30 Protocol Select the application protocol to run on
the RS-485 interface
8-31 Address Set the node address. Note: The address
range depends on the protocol selected in
8-30 Protocol
8-32 Baud Rate Set the baud rate. Note: The default baud
rate depends on the protocol selected in
8-30 Protocol
8-33 Parity / Stop Set the parity and number of stop bits.
Bits Note: The default selection depends on the
protocol selected in 8-30 Protocol
8-35 Minimum Specify a minimum delay time between
Response Delay receiving a request and transmitting a
response, which can be used for
overcoming modem turnaround delays.
8-36 Max Response Specify a maximum delay time between
Delay transmitting a request and receiving a
response.
8-37 Maximum Specify a maximum delay time between
Inter-Char Delay two received bytes to ensure timeout when
transmission is interrupted.
Table 7.2 Modbus Communication Parameters

7.1.3 EMC Precautions

To achieve interference-free operation of the RS-485
network, the following EMC precautions are recommended.
Relevant national and local regulations, regarding
protective earth connection, for example, 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. Normally a distance of 200 mm (8 in) is sufficient.
However, in situations where cables run in parallel over
long distances, keeping the greatest possible distance
between cables is recommended. When crossing is
MG20Z202