Siemens SIMATIC TIWAY 1 User Manual page 203

The receiver checks the BCS by adding all the received frame bytes excluding
the starting and ending flags, and any inserted DLEs. The procedure is:
1. Zero the two calculation registers.
2. Begin with the address field byte.
3. Add the byte to the most significant register including any carry bit
4. Add the most significant register to the least significant register, again
5. Get the next frame byte and go to Step 3. When there are no more bytes,
Example B–4 illustrates the receiver check of the BCS in Example B–3. Notice
that both registers contain negative zero values at the end of the calculation,
indicating a successful frame transmission. (Note: All values are in
hexadecimal notation.)
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USING THE BDLC PROTOCOL
generated. (Use only eight bit arithmetic in these steps).
including the carry.
both calculation registers should be ZERO or a NEGATIVE ZERO
represented by FF hex. If not, then an error has occurred.
Example B–4 Block Checksum Receiver Calculation
Frame
Byte
01
93
D5
95
MSB LSB
00 00
01 01
94 95
6A FF
FF FF
B–18