即S4等于M8异或M9异或M10异或M11异或M12。这样,对于一串码流,知道几位校验位,可以很快确定哪一位出错。而在发送端,可以根据S4,S3,S2,S1的表达式求出P4,P3,P2,P1的表达式,只要把式子右边的P4或P3或P2或P1移到左边替换掉S4或S3或S2或S1就可以了。面举例说明:用^表示异或D7,D6,D5,D4,D3,D2,D1,D0=11010001S4=M8^M9^M10^M11^M12=D7^D6^D5^D4^P4; P4=D7^D6^D5^D4;S3=M4^M5^M6^M7^M12 =D7^D3^D2^D1^P3; P3=D7^D3^D2^D1;S2=M2^M3^M6^M7^M10^M11 =D6^D5^D3^D2^D0^P2; P2=D6^D5^D3^D2^D0;S1=M1^M3^M5^M7^M9^M11=D6^D4^D3^D1^D0^P1; P1=D6^D4^D3^D1^D0;所以,P4=D7^D6^D5^D4=1^1^0^1=1P3=D7^D3^D2^D1=1^0^0^0=1P2= D6^D5^D3^D2^D0=1^0^0^0^1=0 P1=D6^D4^D3^D1^D0=1^1^0^0^1=1故,传送码流为D7,D6,D5,D4,P4,D3,D2,D1,P3,D0,P2,P1等于110110001101若接收端收到110110001101,则S4=M8^M9^M10^M11^M12=1^1^0^1^1=0S3=M4^M5^M6^M7^M12=1^0^0^0^1=0S2=M2^M3^M6^M7^M10^M11=1^0^0^0^1^1=1 S1=M1^M3^M5^M7^M9^M11=1^1^0^0^1^1=0故,接收码流正确。若M6出错,由0变为1。则S4=M8^M9^M10^M11^M12=1^1^0^1^1=0S3=M4^M5^M6^M7^M12=1^0^1^0^1=1S2=M2^M3^M6^M7^M10^M11=0^1^1^1^0^1=0 S1=M1^M3^M5^M7^M9^M11=1^1^0^0^1^1=0