Siemens SIMATIC S7-400 CP 440 - Block Checksum; Figure 2-12 Block Checksum

Basic Principles of Serial Data Transmission
2.5 Data Transmission with the 3964(R) Procedure
Point-to-point connection CP 440 Installation and Parameter Assignment
44 Manual, 09/2007, A5E00057742-03
2.5.2 Block Checksum
Block Checksum
With the 3964(R) transmission protocol, data integrity is increased by the additional sending
of a block check character (BCC).
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Figure 2-12 Block Checksum
The block checksum is the even longitudinal parity (EXOR operation on all data bytes) of a
sent or received block. Its calculation begins with the first byte of user data (first byte of the
message frame) after the connection setup, and ends after the DLE ETX code on connection
release.
Note
If DLE duplication occurs, the DLE code is accounted for twice in the BCC calculation.
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