Siemens SIMATIC CP 340 User Manual page 39

Releasing a Receive Connection
If transmission errors occur during receiving (lost character, frame error, parity error, etc.),
the procedure continues to receive until the connection is shut down, then an NAK is sent to
the communication partner. A repetition is then expected. If the undamaged block still cannot
be received after the number of repeat attempts defined on parameter assignment, or if the
communication partner does not start the repetition within a block wait time of 4 seconds, the
procedure aborts the receive operation. The system program then reports the error to the
function block P_RCV (output parameter STATUS).
When the 3964 procedure detects a DLE ETX character string, it stops receiving and
confirms that the block has been successfully received by sending a DLE character to the
communication partner. When errors are found in the received data, it outputs a NAK signal
to the communication partner. A repetition is then expected.
If the 3964R procedure detects the string DLE ETX BCC, it stops receiving. If the BCC is
correct and no other receive errors have occurred, the CP 340 sends the code DLE to the
communication partner. If the BCC is correct and no other receive errors have occurred, the
3964R procedure sends a DLE and returns to idle mode. If the BCC is faulty or a different
receiving error occurs, an NAK is sent to the communication partner. A repetition is then
expected.
Note
As soon as it is ready, the 3964(R) procedure sends a single NAK to the communication
partner to set the latter to idle.
PtP coupling and configuration of CP 340
Manual, 04/2011, A5E00369892-03
Basic Principles of Serial Data Transmission
2.4 Data Transmission with the 3964(R) Procedure
39