利用netty实现Modbus TCP client/server
READ COILS | 0x01
READ DISCRETE INPUTS | 0x02
READ HOLDING REGISTERS | 0x03
READ INPUT REGISTERS | 0x04
WRITE SINGLE COIL | 0x05
WRITE SINGLE REGISTER | 0x06
WRITE MULTIPLE COILS | 0x0F
WRITE MULTIPLE REGISTERS | 0x10
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