Siemens SIMATIC S7-400 Equipment Manual page 39

Releasing a Send Connection
Once the contents of the buffer have been sent, the procedure adds the codes DLE, ETX and in
the case of 3964(R) only the block checksum BCC as the end code, 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 CP 440 reports the error to the SEND_440
function block (STATUS output parameter).
Point-to-point connection CP 440 Installation and Parameter Assignment
Equipment Manual, 03/2022, A5E00057742-AE
Basic Principles of Serial Data Transmission
2.5 Data Transmission with the 3964(R) Procedure
39