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The Modbus is a serial communication protocol which is published by Modicon Inc.
Originally it is for use with Modicon programmable logic controllers. For more detailed
information about Modbus, please visit http://www.modbus.org/.
SI-W309 Series I/O modules support the Modbus RTU protocol. The communication
Baud Rates is up to 152000 bps. The communication interface is RS-485 with 2-wire
(D+, D-) or 3-wire (D+, D-, ISO.GND). The maximum communication distance is 4000
feet (1.2 km). The command format is as below:
4.1 Command Format
4.1.1 READ Command
Command Format
ID
1 Byte
• ID: The Slave Device address is from 0 to 255.
• CMD: The Modbus commands are as 0x01, 0x02, 0x03...
• Address: The mapping address of register definition for SI-W309
Series.
• CRC: Cyclical Redundancy Check is produced by Modbus utility.
Response
ID
1 Byte
• ID: The Slave Device address is from 0 to 255.
• CMD: The command code is the same as Read command.
• Length: The data length
• Data: The readback data for the CMD
• CRC: Cyclical Redundancy Check is produced by firmware.
Error Response
ID
1 Byte
• ID: The Slave Device address is from 0 to 255.
• CMD: The command code is the same as Read command.
• Address: The address of register definition for SI-Wx09 Series.
• Data: Error Code
• CRC: Cyclical Redundancy Check is produced by firmware.
SI-W309 SERIES USER MANUAL
CMD
Address
1 Byte
2Byte
CMD
Length
1 Byte
CMD
Address
1 Byte
2 Bytes
Chapter 4 MODBUS RTU Protocol
CRC
2Byte
Data
1 Byte
N-Byte
Data
2 Bytes
CRC
2 Bytes
CRC
2 Bytes
Page: 4-2
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