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 A FUNCTION CODE + STARTING A QUANTITY
OF REG 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 A FUNCTION CODE + BYTES COUNT + REGISTERS
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
7