Siemens SIMATIC CP 440 Manual page 45

Sending Data
If a connection is successfully established, the user data contained in the output
buffer of the CP 440 is sent to the communication partner with the selected
transmission parameters. The partner monitors the intervals between incoming
characters. The interval between two characters must not exceed the character
delay time.
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, beginning with connection setup. 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.
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
A5E00057742-02
Basic Principles of Serial Data Transmission
2-27