Supported Modbus functions:
01 (0x01) Read Coils
02 (0x02) Read Discrete Inputs
03 (0x03) Read Holding Registers
04 (0x04) Read Input Registers
05 (0x05) Write Single Coil
06 (0x06) Write Single Register
15 (0x0F) Write Multiple Coils
16 (0x10) Write Multiple registers
20 (0x14) Read File Record
21 (0x15) Write File Record
22 (0x16) Mask Write Register
23 (0x17) Read/Write Multiple registers
43 (0x2B) Read Device Identification