Siemens SIMATIC S7-400 Equipment Manual page 42

Basic Principles of Serial Data Transmission
2.5 Data Transmission with the 3964(R) Procedure
Receiving Data
After a successful connection setup, the receive characters that are arrive are stored in the
receive buffer. If two consecutive DLE codes are received, only one of these is stored in the
receive buffer.
After each receive character, the procedure waits out the character delay time for the next
character. If this period expires before another character is received, an NAK is sent to the
communication partner. The system program reports the error to the RECV_440 function block
(STATUS output parameter). The 3964(R) procedure does not initiate a repetition.
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 transmission attempts defined in the static parameter set, 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 CP 440 reports the first failed transmission and the
final abortion of the receive operation to the RECV_440 function block (STATUS output
parameter).
Releasing a Receive Connection
When the 3964 procedure detects a DLE ETX character string, it ends the receiving operation and
confirms the successfully received block by sending a DLE signal 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 3964(R) procedure recognizes the string DLE ETX BCC, it stops receiving. It then compares
the received block check character with the longitudinal parity calculated internally. If the BCC
is correct and no other receive errors have occurred, the 3964(R) 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.
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Point-to-point connection CP 440 Installation and Parameter Assignment
Equipment Manual, 03/2022, A5E00057742-AE