Try the following:
string sFileName; //file name in DOS format (8.3) byte[] baBody; //byte array of font file string PacketHeader = "! CISDF\r\n{0}\r\n{1}\r\n{2}\r\n"; //************************ int CheckSum = 0; foreach (byte b in baBody) CheckSum += b; PacketHeader = String.Format(PacketHeader, sFileName, Convert.ToString(baBody.Length, 16).PadLeft(8, '0').ToUpper(), Convert.ToString(65536 - (CheckSum % 65536), 16).PadLeft(4, '0').ToUpper()); List<byte> list = new List<byte>(); list.AddRange(Encoding.Default.GetBytes(PacketHeader.ToCharArray())); list.AddRange(baBody); list.AddRange(new byte[] { 0x1B, 0x68, 0x1B, 0x68, 0x1B, 0x68, 0x1B, 0x70, 0x00 }); int PacketSize = list.Count; byte[] tmp; int sourceIndex = 0; while (PacketSize / 512 >= 1) { tmp = list.GetRange(sourceIndex, 512).ToArray(); int BytesSent = bt.WriteBytesToSerialPort(tmp); tmp = null; sourceIndex += BytesSent; PacketSize -= BytesSent; Sleep(100); } if (PacketSize > 0) { tmp = list.GetRange(sourceIndex, list.Count - sourceIndex).ToArray(); PacketSize -= bt.WriteBytesToSerialPort(tmp); tmp = null; } list.Clear(); list = null;