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POXmodem-CRC protocol in c# problem
 

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    <p>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!</p> <p>Here's the CRC code...</p> <pre><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] &lt;&lt; 8) + buf[sz + index + 1]); if (crc == tcrc) { return true; } } else { // use Checksum checking int i; byte cks = 0; for (i = 0; i &lt; 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 &lt; len; counter++) crc = (ushort)((crc &lt;&lt; 8) ^ crc16tab[((crc &gt;&gt; 8) ^ buf[index + counter]) &amp; 0x00FF]); return crc; } #endregion Implementation } </code></pre> <p>Here's the 133 byte array it fails on.</p> <pre><code> [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 </code></pre> <p><strong>UPDATE</strong> 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. </p> <p>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? </p>
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    1. CO`buf[sz + index]` and `buf[sz + index + 1]`. That's as far as I got. You have a method that accepts an array, an offset, and a length and then reads outside those bounds. I can't say that your problem is there, but I can say that you are standing on thin ice.
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    2. COAlso, if you want others to help, you might consider giving us a complete, compilable sample that demonstrates the behavior so that we don't have to reconstruct it ourselves. In doing so you may even find the problem yourself.
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    3. COGood point on the buf issue. I'll fix that. I'll try and get a compilable sample...I'm not sure I'm allowed to post our code out like that, maybe I can do a subset that focuses on this only. I'll see what I can do. Thank you for your help.
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