Modbus RTU Function 06 'Preset Single Register'
To write a word (16 bits) to a register you send the following:
Byte 0 Modbus Device Address (1 to 247)
Byte 1 Function Code / Message Type (0x06)
Byte 2 Register Number (high byte)
Byte 3 Register Number (low byte)
Byte 4 Data0
Byte 5 Data1
Byte 6 CRC of bytes 1 to 6 (high byte)
Byte 7 CRC of bytes 1 to 6 (low byte)
The response is a copy of what was sent
Byte 0 Modbus Device Address
Byte 1 Function Code / Message Type (0x06)
Byte 2 Register Number (high byte)
Byte 3 Register Number (low byte)
Byte 4 Data0
Byte 5 Data1
Byte 6 CRC of bytes 1 to 6 (high byte)
Byte 7 CRC of bytes 1 to 6 (low byte)
