Siemens SIMATIC CP 340 - Handling Errored Data

Basic Principles of Serial Data Transmission
2.4 Data Transmission with the 3964(R) Procedure
PtP coupling and configuration of CP 340
Manual, 04/2011, A5E00369892-03
43
2.4.5 Handling Errored Data
Handling errored data
The figure below illustrates how errored data is handled with the 3964(R) procedure.
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Figure 2-11 Data traffic when receiving errored data
When DLE, ETX, BCC is received, the CP 340 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 340 responds with DLE.
Otherwise, it 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 340 aborts the receive
operation.