WEG MVW3000 A0040 V023, User Manual

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9-24 | MVW3000
9
Communication Networks
9-24 | MVW3000
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The reference signal for the RS-485 interface (SREF) should be used if the master of the network is not referenced
to the ground used in the installation. For instance, in case the master is fed by an isolated power supply, it
is necessary to ground that power supply reference, or take this reference signal to the rest of the system.
Normally, it is only necessary to connect the A (-) and B (+) signals, without the connection of the SREF signal.
RS-232 Serial Interface Module.
The MVW3000 serial interface connection is available at the MVC4 board XC7 connector (refer to the physical
position in
Figure 7.1 on page 7-1 ).
XC7 RS-232
6
5
4
1
2
3
TX
0 V
RX
5 V
0 V
Figure 9.14: XC7 (RJ12) connector signal description
NOTE!
„
The RS-232 wiring must be separated from power cables and 110/220 V command.
„
It is not possible to use RS-232 and RS-485 simultaneously.
9.3 MODBUS-RTU
9.3.1 Introduction to the Modbus-RTU Protocol
The Modbus protocol was initially developed in 1979. Nowadays it is an open protocol, widely spread and used
by many manufacturers in several equipments. The MVW3000 Modbus-RTU communication was developed
based in two documents:
1. MODBUS Protocol Reference Guide Rev. J, MODICON, June 1996.
2. MODBUS Application Protocol Specification, MODBUS.ORG, may 8th 2002.
These documents define the format of the messages used by the elements that compose the Modbus network,
the services (or functions) that can be made available through the network, and how these elements exchange
date in the network.
9.3.1.1 Transmission Modes
Two transmission modes are defined in the protocol specification: ASCII and RTU. The modes define how the
bytes of the message are transmitted. It is not possible to use both transmission modes in the same network.
In the RTU mode each transmitted package has 1 start bit, eight data bits, 1 parity bit (optional) and 1 stop bit (2
stop bits if the parity bit is not used). Therefore, the bit sequence for the transmission of one byte is the following:
Start B0 B1 B2 B3 B4 B5 B6 B7 Parity or Stop Stop
In the RTU mode each data byte is transmitted as being a single word directly with its value in hexadecimal. The
MVW3000 uses only this transmission mode for communication, not having therefore, the ASCII communication
mode.