CRC 16 Calculation

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Introduction

When you communicate with hardware through your system, you usually use Rs232 port to communicate. CRC is used for error checking while sending commands to hardware.

The CRC calculation is done using some algorithm that we will discuss. Before that, here I am going to discuss CRC calculation for Modbus protocol. A two 8bit bytes CRC code must be generated and sent along with other bytes of data which you are going to send through com port.

Here I am going to explain to you how CRC calculation is done in RTU mode for reading the holding register values.

Background

Under frame checking, there are 2 types LRC (Longitudinal Redundancy Check) which are used only when you communicate in ASCII mode. Cyclic Redundancy Check is used while you communicate in RTU mode.

The error checking field of a message frame contains two 8bit bytes that contain the result of the CRC calculation performed on the message contents.

Using the Code

Add CRC_Modbus.cs class to your project. Using namespace CRC, create an object of CRC_Modbus class. There is a method crc16 which calculates and returns two 8bit CRC bytes. In CRC_Modbus constructor values for the CRC Table are initialized:

public CRC_Modbus()
{
            crc_table[0] = 0x0;
            crc_table[1] = 0xC1;
            crc_table[2] = 0x81;
            crc_table[3] = 0x40;
            crc_table[4] = 0x1;
            crc_table[5] = 0xC0;
            crc_table[6] = 0x80;
            crc_table[7] = 0x41;
            crc_table[8] = 0x1;
            crc_table[9] = 0xC0;
            crc_table[10] = 0x80;
            crc_table[11] = 0x41;
            crc_table[12] = 0x0;
            crc_table[13] = 0xC1;
            crc_table[14] = 0x81;
            crc_table[15] = 0x40;
            crc_table[16] = 0x1;
            crc_table[17] = 0xC0;
            crc_table[18] = 0x80;
            crc_table[19] = 0x41;
            crc_table[20] = 0x0;
            crc_table[21] = 0xC1;
            crc_table[22] = 0x81;
            crc_table[23] = 0x40;
            crc_table[24] = 0x0;
            crc_table[25] = 0xC1;
            crc_table[26] = 0x81;
            crc_table[27] = 0x40;
            crc_table[28] = 0x1;
            crc_table[29] = 0xC0;
            crc_table[30] = 0x80;
            crc_table[31] = 0x41;
            crc_table[32] = 0x1;
            crc_table[33] = 0xC0;
            crc_table[34] = 0x80;
            crc_table[35] = 0x41;
            crc_table[36] = 0x0;
            crc_table[37] = 0xC1;
            crc_table[38] = 0x81;
            crc_table[39] = 0x40;
            crc_table[40] = 0x0;
            crc_table[41] = 0xC1;
            crc_table[42] = 0x81;
            crc_table[43] = 0x40;
            crc_table[44] = 0x1;
            crc_table[45] = 0xC0;
            crc_table[46] = 0x80;
            crc_table[47] = 0x41;
            crc_table[48] = 0x0;
            crc_table[49] = 0xC1;
            crc_table[50] = 0x81;
            crc_table[51] = 0x40;
            crc_table[52] = 0x1;
            crc_table[53] = 0xC0;
            crc_table[54] = 0x80;
            crc_table[55] = 0x41;
            crc_table[56] = 0x1;
            crc_table[57] = 0xC0;
            crc_table[58] = 0x80;
            crc_table[59] = 0x41;
            crc_table[60] = 0x0;
            crc_table[61] = 0xC1;
            crc_table[62] = 0x81;
            crc_table[63] = 0x40;
            crc_table[64] = 0x1;
            crc_table[65] = 0xC0;
            crc_table[66] = 0x80;
            crc_table[67] = 0x41;
            crc_table[68] = 0x0;
            crc_table[69] = 0xC1;
            crc_table[70] = 0x81;
            crc_table[71] = 0x40;
            crc_table[72] = 0x0;
            crc_table[73] = 0xC1;
            crc_table[74] = 0x81;
            crc_table[75] = 0x40;
            crc_table[76] = 0x1;
            crc_table[77] = 0xC0;
            crc_table[78] = 0x80;
            crc_table[79] = 0x41;
            crc_table[80] = 0x0;
            crc_table[81] = 0xC1;
            crc_table[82] = 0x81;
            crc_table[83] = 0x40;
            crc_table[84] = 0x1;
            crc_table[85] = 0xC0;
            crc_table[86] = 0x80;
            crc_table[87] = 0x41;
            crc_table[88] = 0x1;
            crc_table[89] = 0xC0;
            crc_table[90] = 0x80;
            crc_table[91] = 0x41;
            crc_table[92] = 0x0;
            crc_table[93] = 0xC1;
            crc_table[94] = 0x81;
            crc_table[95] = 0x40;
            crc_table[96] = 0x0;
            crc_table[97] = 0xC1;
            crc_table[98] = 0x81;
            crc_table[99] = 0x40;
            crc_table[100] = 0x1;
            crc_table[101] = 0xC0;
            crc_table[102] = 0x80;
            crc_table[103] = 0x41;
            crc_table[104] = 0x1;
            crc_table[105] = 0xC0;
            crc_table[106] = 0x80;
            crc_table[107] = 0x41;
            crc_table[108] = 0x0;
            crc_table[109] = 0xC1;
            crc_table[110] = 0x81;
            crc_table[111] = 0x40;
            crc_table[112] = 0x1;
            crc_table[113] = 0xC0;
            crc_table[114] = 0x80;
            crc_table[115] = 0x41;
            crc_table[116] = 0x0;
            crc_table[117] = 0xC1;
            crc_table[118] = 0x81;
            crc_table[119] = 0x40;
            crc_table[120] = 0x0;
            crc_table[121] = 0xC1;
            crc_table[122] = 0x81;
            crc_table[123] = 0x40;
            crc_table[124] = 0x1;
            crc_table[125] = 0xC0;
            crc_table[126] = 0x80;
            crc_table[127] = 0x41;
            crc_table[128] = 0x1;
            crc_table[129] = 0xC0;
            crc_table[130] = 0x80;
            crc_table[131] = 0x41;
            crc_table[132] = 0x0;
            crc_table[133] = 0xC1;
            crc_table[134] = 0x81;
            crc_table[135] = 0x40;
            crc_table[136] = 0x0;
            crc_table[137] = 0xC1;
            crc_table[138] = 0x81;
            crc_table[139] = 0x40;
            crc_table[140] = 0x1;
            crc_table[141] = 0xC0;
            crc_table[142] = 0x80;
            crc_table[143] = 0x41;
            crc_table[144] = 0x0;
            crc_table[145] = 0xC1;
            crc_table[146] = 0x81;
            crc_table[147] = 0x40;
            crc_table[148] = 0x1;
            crc_table[149] = 0xC0;
            crc_table[150] = 0x80;
            crc_table[151] = 0x41;
            crc_table[152] = 0x1;
            crc_table[153] = 0xC0;
            crc_table[154] = 0x80;
            crc_table[155] = 0x41;
            crc_table[156] = 0x0;
            crc_table[157] = 0xC1;
            crc_table[158] = 0x81;
            crc_table[159] = 0x40;
            crc_table[160] = 0x0;
            crc_table[161] = 0xC1;
            crc_table[162] = 0x81;
            crc_table[163] = 0x40;
            crc_table[164] = 0x1;
            crc_table[165] = 0xC0;
            crc_table[166] = 0x80;
            crc_table[167] = 0x41;
            crc_table[168] = 0x1;
            crc_table[169] = 0xC0;
            crc_table[170] = 0x80;
            crc_table[171] = 0x41;
            crc_table[172] = 0x0;
            crc_table[173] = 0xC1;
            crc_table[174] = 0x81;
            crc_table[175] = 0x40;
            crc_table[176] = 0x1;
            crc_table[177] = 0xC0;
            crc_table[178] = 0x80;
            crc_table[179] = 0x41;
            crc_table[180] = 0x0;
            crc_table[181] = 0xC1;
            crc_table[182] = 0x81;
            crc_table[183] = 0x40;
            crc_table[184] = 0x0;
            crc_table[185] = 0xC1;
            crc_table[186] = 0x81;
            crc_table[187] = 0x40;
            crc_table[188] = 0x1;
            crc_table[189] = 0xC0;
            crc_table[190] = 0x80;
            crc_table[191] = 0x41;
            crc_table[192] = 0x0;
            crc_table[193] = 0xC1;
            crc_table[194] = 0x81;
            crc_table[195] = 0x40;
            crc_table[196] = 0x1;
            crc_table[197] = 0xC0;
            crc_table[198] = 0x80;
            crc_table[199] = 0x41;
            crc_table[200] = 0x1;
            crc_table[201] = 0xC0;
            crc_table[202] = 0x80;
            crc_table[203] = 0x41;
            crc_table[204] = 0x0;
            crc_table[205] = 0xC1;
            crc_table[206] = 0x81;
            crc_table[207] = 0x40;
            crc_table[208] = 0x1;
            crc_table[209] = 0xC0;
            crc_table[210] = 0x80;
            crc_table[211] = 0x41;
            crc_table[212] = 0x0;
            crc_table[213] = 0xC1;
            crc_table[214] = 0x81;
            crc_table[215] = 0x40;
            crc_table[216] = 0x0;
            crc_table[217] = 0xC1;
            crc_table[218] = 0x81;
            crc_table[219] = 0x40;
            crc_table[220] = 0x1;
            crc_table[221] = 0xC0;
            crc_table[222] = 0x80;
            crc_table[223] = 0x41;
            crc_table[224] = 0x1;
            crc_table[225] = 0xC0;
            crc_table[226] = 0x80;
            crc_table[227] = 0x41;
            crc_table[228] = 0x0;
            crc_table[229] = 0xC1;
            crc_table[230] = 0x81;
            crc_table[231] = 0x40;
            crc_table[232] = 0x0;
            crc_table[233] = 0xC1;
            crc_table[234] = 0x81;
            crc_table[235] = 0x40;
            crc_table[236] = 0x1;
            crc_table[237] = 0xC0;
            crc_table[238] = 0x80;
            crc_table[239] = 0x41;
            crc_table[240] = 0x0;
            crc_table[241] = 0xC1;
            crc_table[242] = 0x81;
            crc_table[243] = 0x40;
            crc_table[244] = 0x1;
            crc_table[245] = 0xC0;
            crc_table[246] = 0x80;
            crc_table[247] = 0x41;
            crc_table[248] = 0x1;
            crc_table[249] = 0xC0;
            crc_table[250] = 0x80;
            crc_table[251] = 0x41;
            crc_table[252] = 0x0;
            crc_table[253] = 0xC1;
            crc_table[254] = 0x81;
            crc_table[255] = 0x40;
            crc_table[256] = 0x0;
            crc_table[257] = 0xC0;
            crc_table[258] = 0xC1;
            crc_table[259] = 0x1;
            crc_table[260] = 0xC3;
            crc_table[261] = 0x3;
            crc_table[262] = 0x2;
            crc_table[263] = 0xC2;
            crc_table[264] = 0xC6;
            crc_table[265] = 0x6;
            crc_table[266] = 0x7;
            crc_table[267] = 0xC7;
            crc_table[268] = 0x5;
            crc_table[269] = 0xC5;
            crc_table[270] = 0xC4;
            crc_table[271] = 0x4;
            crc_table[272] = 0xCC;
            crc_table[273] = 0xC;
            crc_table[274] = 0xD;
            crc_table[275] = 0xCD;
            crc_table[276] = 0xF;
            crc_table[277] = 0xCF;
            crc_table[278] = 0xCE;
            crc_table[279] = 0xE;
            crc_table[280] = 0xA;
            crc_table[281] = 0xCA;
            crc_table[282] = 0xCB;
            crc_table[283] = 0xB;
            crc_table[284] = 0xC9;
            crc_table[285] = 0x9;
            crc_table[286] = 0x8;
            crc_table[287] = 0xC8;
            crc_table[288] = 0xD8;
            crc_table[289] = 0x18;
            crc_table[290] = 0x19;
            crc_table[291] = 0xD9;
            crc_table[292] = 0x1B;
            crc_table[293] = 0xDB;
            crc_table[294] = 0xDA;
            crc_table[295] = 0x1A;
            crc_table[296] = 0x1E;
            crc_table[297] = 0xDE;
            crc_table[298] = 0xDF;
            crc_table[299] = 0x1F;
            crc_table[300] = 0xDD;
            crc_table[301] = 0x1D;
            crc_table[302] = 0x1C;
            crc_table[303] = 0xDC;
            crc_table[304] = 0x14;
            crc_table[305] = 0xD4;
            crc_table[306] = 0xD5;
            crc_table[307] = 0x15;
            crc_table[308] = 0xD7;
            crc_table[309] = 0x17;
            crc_table[310] = 0x16;
            crc_table[311] = 0xD6;
            crc_table[312] = 0xD2;
            crc_table[313] = 0x12;
            crc_table[314] = 0x13;
            crc_table[315] = 0xD3;
            crc_table[316] = 0x11;
            crc_table[317] = 0xD1;
            crc_table[318] = 0xD0;
            crc_table[319] = 0x10;
            crc_table[320] = 0xF0;
            crc_table[321] = 0x30;
            crc_table[322] = 0x31;
            crc_table[323] = 0xF1;
            crc_table[324] = 0x33;
            crc_table[325] = 0xF3;
            crc_table[326] = 0xF2;
            crc_table[327] = 0x32;
            crc_table[328] = 0x36;
            crc_table[329] = 0xF6;
            crc_table[330] = 0xF7;
            crc_table[331] = 0x37;
            crc_table[332] = 0xF5;
            crc_table[333] = 0x35;
            crc_table[334] = 0x34;
            crc_table[335] = 0xF4;
            crc_table[336] = 0x3C;
            crc_table[337] = 0xFC;
            crc_table[338] = 0xFD;
            crc_table[339] = 0x3D;
            crc_table[340] = 0xFF;
            crc_table[341] = 0x3F;
            crc_table[342] = 0x3E;
            crc_table[343] = 0xFE;
            crc_table[344] = 0xFA;
            crc_table[345] = 0x3A;
            crc_table[346] = 0x3B;
            crc_table[347] = 0xFB;
            crc_table[348] = 0x39;
            crc_table[349] = 0xF9;
            crc_table[350] = 0xF8;
            crc_table[351] = 0x38;
            crc_table[352] = 0x28;
            crc_table[353] = 0xE8;
            crc_table[354] = 0xE9;
            crc_table[355] = 0x29;
            crc_table[356] = 0xEB;
            crc_table[357] = 0x2B;
            crc_table[358] = 0x2A;
            crc_table[359] = 0xEA;
            crc_table[360] = 0xEE;
            crc_table[361] = 0x2E;
            crc_table[362] = 0x2F;
            crc_table[363] = 0xEF;
            crc_table[364] = 0x2D;
            crc_table[365] = 0xED;
            crc_table[366] = 0xEC;
            crc_table[367] = 0x2C;
            crc_table[368] = 0xE4;
            crc_table[369] = 0x24;
            crc_table[370] = 0x25;
            crc_table[371] = 0xE5;
            crc_table[372] = 0x27;
            crc_table[373] = 0xE7;
            crc_table[374] = 0xE6;
            crc_table[375] = 0x26;
            crc_table[376] = 0x22;
            crc_table[377] = 0xE2;
            crc_table[378] = 0xE3;
            crc_table[379] = 0x23;
            crc_table[380] = 0xE1;
            crc_table[381] = 0x21;
            crc_table[382] = 0x20;
            crc_table[383] = 0xE0;
            crc_table[384] = 0xA0;
            crc_table[385] = 0x60;
            crc_table[386] = 0x61;
            crc_table[387] = 0xA1;
            crc_table[388] = 0x63;
            crc_table[389] = 0xA3;
            crc_table[390] = 0xA2;
            crc_table[391] = 0x62;
            crc_table[392] = 0x66;
            crc_table[393] = 0xA6;
            crc_table[394] = 0xA7;
            crc_table[395] = 0x67;
            crc_table[396] = 0xA5;
            crc_table[397] = 0x65;
            crc_table[398] = 0x64;
            crc_table[399] = 0xA4;
            crc_table[400] = 0x6C;
            crc_table[401] = 0xAC;
            crc_table[402] = 0xAD;
            crc_table[403] = 0x6D;
            crc_table[404] = 0xAF;
            crc_table[405] = 0x6F;
            crc_table[406] = 0x6E;
            crc_table[407] = 0xAE;
            crc_table[408] = 0xAA;
            crc_table[409] = 0x6A;
            crc_table[410] = 0x6B;
            crc_table[411] = 0xAB;
            crc_table[412] = 0x69;
            crc_table[413] = 0xA9;
            crc_table[414] = 0xA8;
            crc_table[415] = 0x68;
            crc_table[416] = 0x78;
            crc_table[417] = 0xB8;
            crc_table[418] = 0xB9;
            crc_table[419] = 0x79;
            crc_table[420] = 0xBB;
            crc_table[421] = 0x7B;
            crc_table[422] = 0x7A;
            crc_table[423] = 0xBA;
            crc_table[424] = 0xBE;
            crc_table[425] = 0x7E;
            crc_table[426] = 0x7F;
            crc_table[427] = 0xBF;
            crc_table[428] = 0x7D;
            crc_table[429] = 0xBD;
            crc_table[430] = 0xBC;
            crc_table[431] = 0x7C;
            crc_table[432] = 0xB4;
            crc_table[433] = 0x74;
            crc_table[434] = 0x75;
            crc_table[435] = 0xB5;
            crc_table[436] = 0x77;
            crc_table[437] = 0xB7;
            crc_table[438] = 0xB6;
            crc_table[439] = 0x76;
            crc_table[440] = 0x72;
            crc_table[441] = 0xB2;
            crc_table[442] = 0xB3;
            crc_table[443] = 0x73;
            crc_table[444] = 0xB1;
            crc_table[445] = 0x71;
            crc_table[446] = 0x70;
            crc_table[447] = 0xB0;
            crc_table[448] = 0x50;
            crc_table[449] = 0x90;
            crc_table[450] = 0x91;
            crc_table[451] = 0x51;
            crc_table[452] = 0x93;
            crc_table[453] = 0x53;
            crc_table[454] = 0x52;
            crc_table[455] = 0x92;
            crc_table[456] = 0x96;
            crc_table[457] = 0x56;
            crc_table[458] = 0x57;
            crc_table[459] = 0x97;
            crc_table[460] = 0x55;
            crc_table[461] = 0x95;
            crc_table[462] = 0x94;
            crc_table[463] = 0x54;
            crc_table[464] = 0x9C;
            crc_table[465] = 0x5C;
            crc_table[466] = 0x5D;
            crc_table[467] = 0x9D;
            crc_table[468] = 0x5F;
            crc_table[469] = 0x9F;
            crc_table[470] = 0x9E;
            crc_table[471] = 0x5E;
            crc_table[472] = 0x5A;
            crc_table[473] = 0x9A;
            crc_table[474] = 0x9B;
            crc_table[475] = 0x5B;
            crc_table[476] = 0x99;
            crc_table[477] = 0x59;
            crc_table[478] = 0x58;
            crc_table[479] = 0x98;
            crc_table[480] = 0x88;
            crc_table[481] = 0x48;
            crc_table[482] = 0x49;
            crc_table[483] = 0x89;
            crc_table[484] = 0x4B;
            crc_table[485] = 0x8B;
            crc_table[486] = 0x8A;
            crc_table[487] = 0x4A;
            crc_table[488] = 0x4E;
            crc_table[489] = 0x8E;
            crc_table[490] = 0x8F;
            crc_table[491] = 0x4F;
            crc_table[492] = 0x8D;
            crc_table[493] = 0x4D;
            crc_table[494] = 0x4C;
            crc_table[495] = 0x8C;
            crc_table[496] = 0x44;
            crc_table[497] = 0x84;
            crc_table[498] = 0x85;
            crc_table[499] = 0x45;
            crc_table[500] = 0x87;
            crc_table[501] = 0x47;
            crc_table[502] = 0x46;
            crc_table[503] = 0x86;
            crc_table[504] = 0x82;
            crc_table[505] = 0x42;
            crc_table[506] = 0x43;
            crc_table[507] = 0x83;
            crc_table[508] = 0x41;
            crc_table[509] = 0x81;
            crc_table[510] = 0x80;
            crc_table[511] = 0x40;
 }

CRC calculation method is as follows:

public int crc16(byte[] modbusframe,int Length)
 {
        int i;
        int index;
        int crc_Low = 0xFF;
        int crc_High = 0xFF;

        for (i = 0; i<Length; i++ )
        {
             index = crc_High ^ (char)modbusframe[i];
             crc_High = crc_Low ^ crc_table[index];
             crc_Low = (byte)crc_table[index+ 256];                             
        }
        
        return crc_High*256+crc_Low;        
 } 

6 bytes data is passed for modbusframe:

byte[] modbus_bytes=new byte[8];

byte device_address=Convert.ToByte(txt_device_address.Text);
byte data_type=0x03; // for reading holding registers
Int64 start_address=Convert.ToInt64(txt_start_address.Text);
start_address=start_address -1;
            
Int64 start_address_higer=start_address/256;
Int64 start_address_lower=start_address % 256;
            
byte length=Convert.ToByte(txt_length.Text);

modbus_bytes[0]=device_address;
modbus_bytes[1]=data_type;
modbus_bytes[2]=(byte)start_address_higer;
modbus_bytes[3]=(byte)start_address_lower;
modbus_bytes[4]=0x00;
modbus_bytes[5]=length;

int crc_result=CRC.crc16(modbus_bytes,6);
byte[] crc_bytes=BitConverter.GetBytes(crc_result);
            
modbus_bytes[6]=crc_bytes[1];
modbus_bytes[7]=crc_bytes[0];

com.Output=modbus_bytes;  // Send byte data through com port.

Points of Interest

It's really crazy to calculate CRC code for me and it depends on your interest in using this code to generate CRC Code.

History

• 16^th June, 2007: Initial post