Optidrive VTC – User Guide
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10.
Modbus Communications
10.1.
Introduction
The Optidrive VTC can be connected to a Modbus RTU network via the RJ11 connector on the front of the drive.
10.2.
Modbus RTU Specification
Protocol
Modbus RTU
Error check
CRC
Baud rate
9600bps, 19200bps, 38400bps, 57600bps, 115200bps (default)
Data format
1 start bit, 8 data bits, 1 stop bits, no parity.
Physical signal
RS 485 (2-wire)
User interface
RJ11 (see section 4.4 for more information)
10.3.
RJ11 Connector Configuration
10.4.
Modbus Telegram Structure
The Optidrive VTC supports Master / Slave Modbus RTU communications, using the 03 Read Holding Registers and 06 Write Single Holding Register
commands. Many Master devices treat the first Register address as Register 0; therefore it may be necessary to convert the Register Numbers
detail in section 10.5 by subtracting 1 to obtain the correct Register address. The telegram structure is as follows:-
Command 03 – Read Holding Registers
Command 06 – Write Single Holding Register
Master Telegram
Length
Slave Response
Length
Master Telegram
Length
Slave Response
Length
Slave Address
1 Byte
Slave Address
1 Byte
Slave Address
1 Byte
Slave Address
1 Byte
Function Code (03)
1 Byte
Starting Address
1 Byte
Function Code (06)
1 Byte
Function Code (06)
1 Byte
1
st
Register Address
2 Bytes
1
st
Register Value
2 Bytes
Register Address
2 Bytes
Register Address
2 Bytes
No. Of Registers
2 Bytes
2
nd
Register Value
2 Bytes
Value
2 Bytes
Register Value
2 Bytes
CRC Checksum
2 Bytes
Etc...
CRC Checksum
2 Bytes
CRC Checksum
2 Bytes
CRC Checksum
2 Bytes
