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Checksum calculation

This checksum is the same as the IP checksum.

Definition: sum of 1’s complement of all 16 bits words of whole message

(FiveCo packet) except checksum bytes.

Note: all values are unsigned!

Sequence:

1. Clear accumulator

Loop

x. Only if last word is not made of two bytes, the data byte is the upper byte (big endian)

2. Compute 1’s complement of each 16bits word, result is 16bits

3. Convert last result from 16 bits to 32 bits, result is 32bits: 0x0000+last result

4. Add last result to the 32 bits accumulator

Try the Loop

5. Convert accumulator in two 16bits words

6. Add those two 16bits words, result is 16bits word.

7. If an overflow occurs with the last addition (Carry), add 1 to the last result.

8. Last result is the final result

Example (in hexadecimal):

!0x0021 (0XFFDE) Æ 0x0000FFDE (Read)

+!0x1234 (0xEDCB) Æ 0x0001EDA9 (TransID)

+!0x0003 (0xFFFC) Æ 0x0002EDA5 (3 reg to read)

+!0x0A10 (0XF5EF) Æ 0x0003E394 (reg 0A,10,02)

+!0x02(00)(0XFDFF) Æ 0x0004E193

Note that in this case a last 00 is implicitly used. (02 Æ 02 00).

0x0004 + 0xE193 = 0xE197, (carry=0)

xE197 + carry = 0xE197

Checksum = 0xE197

FMod-TCP User Manual v.2.8

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Philips FMod-TCP BOX Manual de usuario Pagina 20