private byte[] CreateReadHeaderRTU(ushort id, byte unit, ushort startAddress, ushort length, byte function)
 {
     byte[] data = new byte[6];
     byte[] crc = new byte[2];
     data[0] = unit; // Was 0 Slave address
     data[1] = function; // Function code
     byte[] _adr = BitConverter.GetBytes((short) IPAddress.HostToNetworkOrder((short) startAddress));
     data[2] = _adr[0]; // Start address
     data[3] = _adr[1]; // Start address
     byte[] _length = BitConverter.GetBytes((short) IPAddress.HostToNetworkOrder((short) length));
     data[4] = _length[0]; // Number of data to read
     data[5] = _length[1]; // Number of data to read
     crc = ModbusUtil.CalculateCrc(data);
     byte[] ret = new byte[data.Length + crc.Length];
     Buffer.BlockCopy(data, 0, ret, 0, data.Length);
     Buffer.BlockCopy(crc, 0, ret, data.Length, crc.Length);
     return ret;
 }

//TCP 00000000 | 00 00 00 00 00 07 01 03 04 42 A0 9B 1C          | .........B...
//RTU 00000000 | 03 03 04 44 C2 41 88 5C C9                      | ...D.A.\.
ushort rtu = BitConverter.ToUInt16(tcpAsyClBuffer, resultptr+2); // Zero for TCP, bytecount for RTU

grpData.Text = "Read holding register";
globaldata = values;  // sometimes goes NULL