Siemens SIMATIC CP 440, Руководство пользователя

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Basic Principles of Serial Data Transmission
2-31
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Initialization Conflict
The figure below illustrates the transmission sequence during an initialization
conflict.
Connection
setup
Communication partner
(higher priority) CP 440
(low priority)
Connection
cleardown
User
data
Connection
setup
Start code (02H)
Start code (02H)
Pos. acknowledgment (10H)
1st data byte
2nd data byte
nth data byte
End code (10H)
End code (03H)
3964(R) only
Pos. acknowledgment (10H)
2nd setup attempt
Start code (02H)
Pos. acknowledgment (10H)
STX
STX
DLE
1st byte
2nd byte
nth byte
DLE
ETX
BCC
DLE
STX
DLE
Figure 2-16 Data Traffic during an Initialization Conflict
If a device responds to the communication partner’s send job (code STX) within
the acknowledgment delay time (ADT) by sending the code STX instead of the
acknowledgment DLE or NAK, an initialization conflict occurs. Both devices want to
execute a send job. The device with the lower priority withdraws its send job and
responds with the code DLE. The device with the higher priority sends its data in
the manner described above. Once the connection has been cleared down, the
lower-priority device can execute its send job.
To be able to resolve the initialization conflict, you must parameterize the
communication partners with different priorities.