Siemens SIMATIC TIWAY 1 - Manual (Page 203)

USING THE BDLC PROTOCOL
UNILINK HOST ADAPTER B–18
USER MANUAL
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
generated. (Use only eight bit arithmetic in these steps).
4. Add the most significant register to the least significant register, again
including the carry.
5. Get the next frame byte and go to Step 3. When there are no more bytes,
both calculation registers should be ZERO or a NEGATIVE ZERO
represented by FF hex. If not, then an error has occurred.
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.)
Example B–4 Block Checksum Receiver Calculation
Frame
Byte
MSB LSB
01
00
01 00
01
93 94 95
D5 6A FF
95 FF FF