Siemens SIMATIC S7-400 CP 440 - Handling Corrupt Data

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
Manual, 09/2007, A5E00057742-03
53
2.5.5 Handling Corrupt Data
Reaction to Corrupt Data
The figure below illustrates how corrupt data is handled with the 3964(R) procedure.
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Figure 2-18 Data traffic when receiving corrupt data
When DLE, ETX, BCC is 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 an NAK and waits the block wait time (T) of 4 seconds
for a new attempt. If after the defined 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.
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