Fletcher Checksum; Calculation - Honeywell IPT User Manual

6 Fletcher Checksum

6.1 Calculation

The Fletcher checksum calculation results in two sums:
SUM1[R-1] = D[0] + D[1] + ...D[R-1]
SUM2[R-1] = SUM1[0] + SUM1[1] + ...SUM1[R-1]
where R = number of bytes in the EEPROM storage area from 00 through B5 (182d), including the
check bytes, and where all additions are modulo 256.
If no errors are found, SUM1[R-1] = SUM2[R-1] = 0
Example: 4 bytes of data, 2 check bytes, no errors
Data
Data
Data
Data
Check Byte #1
Check Byte #2
Example with single-bit error
Data
Data
Data
Data
Check Byte #1
Check Byte #2
Example with two single-bit errors
Data
Data
Data
Data
Check Byte #1
Check Byte #2
Example with multiple errors
Data
Data
Data
Data
Check Byte #1
Check Byte #2
Hex Binary Decimal
C0 11000000 192
E7 11100111 231
E5 11100101 229
04 00000100 4
ED 11101101 237
83 10000011 131
Hex Binary Decimal
C0 11000000 192
E7 11100111 231
C5 11000101 197
04 00000100 4
ED 11101101 237
83 10000011 131
Hex Binary Decimal
C2 11000010 194
E5 11100101 229
E5 11100101 229
04 00000100 4
ED 11101101 237
83 10000011 131
Hex Binary Decimal
64 01100100
100
E7 11100111 231
E5 11100101 229
BC 10111100
188
ED 11101101 237
83 10000011 131
SUM1 SUM2
192 192
167 103
140 243
144 131
125 0
0 0
SUM1 SUM2
192 192
167 103
108 211
112 67
93 160
224 128
SUM1 SUM2
194 194
167 105
140 245
144 133
125 2
0 2
SUM1 SUM2
100 100
75 175
48 223
236 203
217 164
92 0
21