// SELECT CARRD METHOD FIX1
        public void SELECTcard()
{

         byte[] buff = new byte[5];
         buff[0] = 4; //áðî¼ íà bytes âî buff
         buff[1] = 186;
         //BAh
         buff[2] = 2;
         //len(bytes)
         buff[3] = 1;
         //01h
         buff[4] = buff[1] ^ buff[2] ^ buff[3]; // the error is here //please can you help
         //checksum
         sndCARD(buff);

}
//Error 1 Cannot implicitly convert type 'int' to 'byte'. An explicit conversion exists (are you missing a cast?)

Your error gives you your answer ...

buff[4] = (byte)(buff[1]^buff[2]^buff[3]);

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