// use modbus rtu
var session = ModbusSessionFactory.CreateRtuSession(serialPort);
// arguments are slave id, data address and number of registers
var builder = RequestBuilderFactory.CreateRequest03<RtuRequest>(1, 0, 4);
// send request and wait for response
var response = await session.SendRequestAsync<ResponseFunc03>(builder);

[StructLayout(LayoutKind.Sequential, Pack = 1)]
private struct ResponseFunc03
{
    public byte numberOfBytes;

    [Endian(Endianness.BigEndian)]
    public short value;

    [Endian(Endianness.BigEndian)]
    public ushort dotPosition;

    public short padding1;
    public short padding2;
    
    //[MarshalAs(UnmanagedType.ByValArray, ArraySubType = UnmanagedType.U2, SizeConst = 4)]
    //[Endian(Endianness.BigEndian)]
    //public ushort[] bytes;
}

 var builder = new RtuRequest.Builder()
    .SetSlaveAddress(0x01)
    .SetFunctionCode(0x09)
    .AddInt16(33)
    .AddInt16(93);
    //.SetObject(requestDataStruct)