Honeywell IPT, User Manual, Page: 21 / 23

21
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
Hex Binary Decimal SUM1 SUM2
Data C0 11000000 192 192 192
Data E7 11100111 231 167 103
Data E5 11100101 229 140 243
Data 04 00000100 4 144 131
Check Byte #1 ED 11101101 237 125 0
Check Byte #2 83 10000011 131 0 0
Example with single-bit error
Hex Binary Decimal SUM1 SUM2
Data C0 11000000 192 192 192
Data E7 11100111 231 167 103
Data C5 11 0 00101 197 108 211
Data 04 00000100 4 112 67
Check Byte #1 ED 11101101 237 93 160
Check Byte #2 83 10000011 131
224 128
Example with two single-bit errors
Hex Binary Decimal SUM1 SUM2
Data C2 110000 1 0 194 194 194
Data E5 111001 0 1 229 167 105
Data E5 11100101 229 140 245
Data 04 00000100 4 144 133
Check Byte #1 ED 11101101 237 125 2
Check Byte #2 83 10000011 131 0 2
Example with multiple errors
Hex Binary Decimal SUM1 SUM2
Data 64 0 1 1 00 1 00 100 100 100
Data E7 11100111 231 75 175
Data E5 11100101 229 48 223
Data BC 1 0 111 100 188 236 203
Check Byte #1 ED 11101101 237 217 164
Check Byte #2 83 10000011 131 92 0