Siemens Simatic S7-300 CP 341 User Manual page 41

Connection release for sending
Once the contents of the buffer have been sent, the procedure adds the DLE and ETX
characters and, with 3964R only, the block checksum BCC as the end identifier, and waits
for an acknowledgment character. If the communication partner sends the DLE character
within the acknowledgment delay time, it means that the data block has been received
without errors. If the communication partner responds with NAK, any other character (except
DLE), or a damaged character, 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 assigned number of attempts to send the data block, the procedure stops trying and
sends a NAK to the communication partner. The CP 341 reports the error to the
P_SND_RK function block (STATUS output parameter).
CP 341 Point-to-Point Communication, Installation and Parameter Assignment
Manual, 09/2008, A5E02191071-01
www.gkbpx.com
Basic Principles of Serial Data Transmission
2.4 Data Transmission with the 3964(R) Procedure
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