Siemens SIMATIC CP 440, Руководство пользователя

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Basic Principles of Serial Data Transmission
2-25
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
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 Data DLE ETX BCC
02H 30H 31H 32H 10H 03H 20H
30 = 0011 0000
31 = 0011 0001
XOR = 0000 0001
32 = 0011 0010
XOR = 0011 0011
10 = 0001 0000
XOR = 0010 0011
03 = 0000 0011
XOR = 0010 0000
BCC 2 0
Figure 2-12 Block Checksum
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.