I'm porting a C++ xmodem protocol to C# and I'm having an issue with the CRC check. The application uploads files through a modem using the xmodem 128 byte protocol with CRC. I test it using Hyperterminal. I can upload small files fine, but when I try larger files (50K +) the CRC always breaks when the low and high bytes are 255. Can someone help me with this? Thanks!
Here's the CRC code...
..
if (check(true, _data, 3, 133))
{
_currentState = State.Good;
}
else
{
_currentState = State.Bad;
}
...
private bool check(bool isCRC, byte[] buf, int index, int sz)
{
if (isCRC) // use CRC checking
{
ushort crc = CRC16.CRC16_ccitt(buf, index, sz);
ushort tcrc = (ushort)((buf[sz + index] << 8) + buf[sz + index + 1]);
if (crc == tcrc)
{
return true;
}
}
else
{ // use Checksum checking
int i;
byte cks = 0;
for (i = 0; i < sz; ++i)
{
cks += buf[i + index];
}
if (cks == buf[sz + index])
return true;
}
return false;
}
internal static class CRC16
{
#region Members
//size = 256
private static ushort[] crc16tab = {
0x0000,0x1021,0x2042,0x3063,0x4084,0x50a5,0x60c6,0x70e7,
0x8108,0x9129,0xa14a,0xb16b,0xc18c,0xd1ad,0xe1ce,0xf1ef,
0x1231,0x0210,0x3273,0x2252,0x52b5,0x4294,0x72f7,0x62d6,
0x9339,0x8318,0xb37b,0xa35a,0xd3bd,0xc39c,0xf3ff,0xe3de,
0x2462,0x3443,0x0420,0x1401,0x64e6,0x74c7,0x44a4,0x5485,
0xa56a,0xb54b,0x8528,0x9509,0xe5ee,0xf5cf,0xc5ac,0xd58d,
0x3653,0x2672,0x1611,0x0630,0x76d7,0x66f6,0x5695,0x46b4,
0xb75b,0xa77a,0x9719,0x8738,0xf7df,0xe7fe,0xd79d,0xc7bc,
0x48c4,0x58e5,0x6886,0x78a7,0x0840,0x1861,0x2802,0x3823,
0xc9cc,0xd9ed,0xe98e,0xf9af,0x8948,0x9969,0xa90a,0xb92b,
0x5af5,0x4ad4,0x7ab7,0x6a96,0x1a71,0x0a50,0x3a33,0x2a12,
0xdbfd,0xcbdc,0xfbbf,0xeb9e,0x9b79,0x8b58,0xbb3b,0xab1a,
0x6ca6,0x7c87,0x4ce4,0x5cc5,0x2c22,0x3c03,0x0c60,0x1c41,
0xedae,0xfd8f,0xcdec,0xddcd,0xad2a,0xbd0b,0x8d68,0x9d49,
0x7e97,0x6eb6,0x5ed5,0x4ef4,0x3e13,0x2e32,0x1e51,0x0e70,
0xff9f,0xefbe,0xdfdd,0xcffc,0xbf1b,0xaf3a,0x9f59,0x8f78,
0x9188,0x81a9,0xb1ca,0xa1eb,0xd10c,0xc12d,0xf14e,0xe16f,
0x1080,0x00a1,0x30c2,0x20e3,0x5004,0x4025,0x7046,0x6067,
0x83b9,0x9398,0xa3fb,0xb3da,0xc33d,0xd31c,0xe37f,0xf35e,
0x02b1,0x1290,0x22f3,0x32d2,0x4235,0x5214,0x6277,0x7256,
0xb5ea,0xa5cb,0x95a8,0x8589,0xf56e,0xe54f,0xd52c,0xc50d,
0x34e2,0x24c3,0x14a0,0x0481,0x7466,0x6447,0x5424,0x4405,
0xa7db,0xb7fa,0x8799,0x97b8,0xe75f,0xf77e,0xc71d,0xd73c,
0x26d3,0x36f2,0x0691,0x16b0,0x6657,0x7676,0x4615,0x5634,
0xd94c,0xc96d,0xf90e,0xe92f,0x99c8,0x89e9,0xb98a,0xa9ab,
0x5844,0x4865,0x7806,0x6827,0x18c0,0x08e1,0x3882,0x28a3,
0xcb7d,0xdb5c,0xeb3f,0xfb1e,0x8bf9,0x9bd8,0xabbb,0xbb9a,
0x4a75,0x5a54,0x6a37,0x7a16,0x0af1,0x1ad0,0x2ab3,0x3a92,
0xfd2e,0xed0f,0xdd6c,0xcd4d,0xbdaa,0xad8b,0x9de8,0x8dc9,
0x7c26,0x6c07,0x5c64,0x4c45,0x3ca2,0x2c83,0x1ce0,0x0cc1,
0xef1f,0xff3e,0xcf5d,0xdf7c,0xaf9b,0xbfba,0x8fd9,0x9ff8,
0x6e17,0x7e36,0x4e55,0x5e74,0x2e93,0x3eb2,0x0ed1,0x1ef0
};
#endregion Members
#region Implementation
static public ushort CRC16_ccitt(byte[] buf, int index, int len)
{
int counter;
ushort crc = 0;
for (counter = 0; counter < len; counter++)
crc = (ushort)((crc << 8) ^ crc16tab[((crc >> 8) ^ buf[index + counter]) & 0x00FF]);
return crc;
}
#endregion Implementation
}
Here's the 133 byte array it fails on.
[0] 1 byte
[1] 127 byte
[2] 128 byte
[3] 84 byte
[4] 52 byte
[5] 51 byte
[6] 49 byte
[7] 48 byte
[8] 50 byte
[9] 51 byte
[10] 57 byte
[11] 50 byte
[12] 48 byte
[13] 70 byte
[14] 48 byte
[15] 48 byte
[16] 48 byte
[17] 48 byte
[18] 48 byte
[19] 48 byte
[20] 48 byte
[21] 48 byte
[22] 48 byte
[23] 48 byte
[24] 84 byte
[25] 32 byte
[26] 32 byte
[27] 32 byte
[28] 32 byte
[29] 32 byte
[30] 32 byte
[31] 32 byte
[32] 32 byte
[33] 32 byte
[34] 32 byte
[35] 32 byte
[36] 32 byte
[37] 32 byte
[38] 32 byte
[39] 32 byte
[40] 32 byte
[41] 32 byte
[42] 32 byte
[43] 32 byte
[44] 32 byte
[45] 32 byte
[46] 32 byte
[47] 32 byte
[48] 32 byte
[49] 32 byte
[50] 32 byte
[51] 32 byte
[52] 32 byte
[53] 32 byte
[54] 32 byte
[55] 32 byte
[56] 32 byte
[57] 32 byte
[58] 32 byte
[59] 32 byte
[60] 32 byte
[61] 32 byte
[62] 32 byte
[63] 32 byte
[64] 32 byte
[65] 32 byte
[66] 32 byte
[67] 32 byte
[68] 32 byte
[69] 48 byte
[70] 49 byte
[71] 45 byte
[72] 32 byte
[73] 32 byte
[74] 32 byte
[75] 32 byte
[76] 32 byte
[77] 32 byte
[78] 32 byte
[79] 32 byte
[80] 32 byte
[81] 32 byte
[82] 13 byte
[83] 10 byte
[84] 80 byte
[85] 48 byte
[86] 51 byte
[87] 66 byte
[88] 90 byte
[89] 66 byte
[90] 32 byte
[91] 53 byte
[92] 55 byte
[93] 54 byte
[94] 53 byte
[95] 53 byte
[96] 48 byte
[97] 48 byte
[98] 48 byte
[99] 48 byte
[100] 84 byte
[101] 52 byte
[102] 51 byte
[103] 77 byte
[104] 79 byte
[105] 51 byte
[106] 55 byte
[107] 49 byte
[108] 56 byte
[109] 48 byte
[110] 48 byte
[111] 48 byte
[112] 48 byte
[113] 48 byte
[114] 48 byte
[115] 48 byte
[116] 48 byte
[117] 48 byte
[118] 48 byte
[119] 56 byte
[120] 57 byte
[121] 52 byte
[122] 48 byte
[123] 52 byte
[124] 57 byte
[125] 32 byte
[126] 32 byte
[127] 32 byte
[128] 32 byte
[129] 32 byte
[130] 32 byte
[131] 255 byte
[132] 255 byte
UPDATE I failed to mention that I'm not reading from a serial port. We have a modem bank that receives calls and opens a connection to our service using TCP/IP. So when I read the bytes I'm reading from a TcpClients stream.
I also noticed that when I send a file to my service using Hyperterminal, packet 255 has a packet number of 255 and a compliment of 255. The compliment should be 0, right?
I figured out the issue I was having. First, my setup. We have a modem bank that takes incoming calls and routes them using telnet TCP/IP to a service running on a windows box. The problem I was having was every time a 255 byte was sent by the client it was preceded by a 255 byte. Since 255 is a valid byte for xmodem packet numbers, packet number compliments and CRC it because an issue when it sent two of them. The reason it was sending two 255 bytes was because Telnet TCP/IP uses byte 255. So, when a 255 byte needs to be sent Telnet will escape it with another 255 byte. This would have never happened if I was working over a serial connection. Since our Modem bank connects to our service using telnet TCP/IP we need to handle the 255 bytes that get escaped. Once I did that everything worked great. I used Hans Passant and Tergiver's advice to fix the errors in my xmodem code, so thank you very much to them for their help.