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7.4 Modbus Functions
The following Modbus functions are supported by the XVG/eXVG:
- Read Registers (0x03)
- Write Single Register (0x06)
- Write Multiple Registers (0x10)
- Diagnostics (0x08) Query Data sub-function (0x00)
The master can address individual slaves, or can initiate a broadcast message (using
device address 0) to all slaves. Slaves perform the actions requested by the broadcast
messages but will not return a response. Per Modbus protocol, only Write Single
Register (0x06) and Write Multiple Registers (0x10) support broadcast message.
7.4.1 Read Registers (0x03)
Request message:
SLAVE ADDRESS + FUNCTION CODE + STARTING ADDRESS + QUANTITY
OF REGISTERS + CRC
Request Length Valid Value
Slave Address 1 Byte 0 to 247
Function code 1 Byte
0x03
Starting Address 2 Bytes
0x0000 to 0xFFFF
Quantity of Registers 2 Bytes
1 to 125 (0x7D)
CRC 2 Bytes
0x0000 to 0xFFFF
Table 4: Modbus Function 03 – Request Message
Response message:
SLAVE ADDRESS + FUNCTION CODE + BYTES COUNT + REGISTERS
VALUES + CRC
Response Length Valid Value
Slave Address 1 Byte 0 to 247
Function code 1 Byte
0x03
Byte count 1 Byte
2xN
Registers Values 2xN Bytes
value
CRC 2 Bytes
0x0000 to 0xFFFF
*N = Quantity of Registers
Table 5: Modbus Function 03 - Response Message
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