Siemens SIMATIC CP 440 Manual page 43

Priority
With the 3964(R) procedure, one communication partner must be assigned a
higher priority and the other partner a lower priority. If both partners begin
connection setup at the same time, the partner with the lower priority will defer its
send job.
Block Check Sum
With the 3964(R) transmission protocol, data integrity is increased by the additional
sending of a block check character (BCC).
Message frame
STX
02H
Figure 2-12
The block checksum is the even longitudinal parity (EXOR logic operation of 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 cleardown.
Note
If DLE duplication occurs, the DLE code is accounted for twice in the BCC
calculation.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Data
30H 31H 32H
30 = 0011
31 = 0011
XOR = 0000
32 = 0011
XOR = 0011
10 = 0001
XOR = 0010
03 = 0000
XOR = 0010
BCC 2
Block Checksum
Basic Principles of Serial Data Transmission
DLE ETX BCC
10H 03H 20H
0000
0001
0001
0010
0011
0000
0011
0011
0000
0
2-25