Siemens SIMATIC CP 440 Manual page 48

Basic Principles of Serial Data Transmission
Handling Errored Data
The figure below illustrates how errored data is handled with the 3964(R)
procedure.
Communication
partner
Start code (02H)
Pos. acknowledgment (10H)
1st data byte
nth data byte
End code (10H)
End code (03H)
3964(R) only
Neg. acknowledgment (15H)
Figure 2-15
When DLE, ETX and BCC are received, the CP 440 compares the BCC of the
communication partner with its own internally calculated value. If the BCC is
correct and no other receive errors occur, the CP 440 responds with DLE.
Otherwise, the CP 440 responds with NAK and waits the block wait time (T) of
4 seconds for a new attempt. If, after the parameterized number of transmission
attempts, the block cannot be received, or if no further attempt is made within the
block wait time, the CP 440 aborts the receive operation.
2-30
New setup attempt
Data Traffic When Receiving Errored Data
Point-to-point connection CP 440 Installation and Parameter Assignment
Receiving data
STX
DLE
1st byte
nth byte
DLE
ETX
BCC
NAK
T
CP 440
Connection
setup
User
data
Connection
cleardown
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