Siemens SIMATIC CP 340 User Manual page 35

Releasing a Send Connection
If the communication partner sends the NAK control code during an active send operation,
the procedure aborts its transmission of the block and tries again as described above. If a
different code is sent, the procedure first waits for the character delay time to expire and then
sends the NAK code to change the mode of the communication partner to idle. Then the
procedure starts to send the data again with the connection setup STX.
Once the contents of the buffer have been sent, the procedure adds the codes DLE, ETX
and with the 3964R only the block checksum BCC as the end identifier, and waits for an
acknowledgment code. If the communication partner sends the DLE code within the
acknowledgment delay time, the data block has been received without errors. If the
communication partner responds with NAK, any other code (except DLE), or a damaged
code, or if the acknowledgment delay time expires without a response, the procedure starts
to send the data again with the connection setup STX.
After the defined number of attempts to send the data block, the procedure stops trying and
sends an NAK to the communication partner. The system program reports the error to the
function block P_SEND (output parameter STATUS).
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
35