Siemens Simatic S7-300 CP 341 User Manual page 43

Receiving data
After a successful connection setup, the receive characters that arrive are stored in the
receive buffer. If two consecutive DLE characters 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, the NAK character is
sent to the communication partner. The system program reports the error to the
P_RCV_RK 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 released, then a NAK is sent to the
communication partner. A repetition is then expected. If the block still cannot be received
without errors 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 cancels the receive operation. The CP 341 reports the first
erroneous transmission and the final cancelation in the P_RCV_RK function block (STATUS
output parameter).
Connection release for receiving
When the 3964 procedure detects a DLE ETX character string, it stops receiving and
confirms that the block has been successfully received by sending a DLE character to the
communication partner. If an error occurs while receiving, the procedure sends a NAK
character to the communication partner. A repetition is then expected.
If the 3964R procedure detects the string DLE ETX BCC, it stops receiving. It compares the
received BCC block check character with the internally calculated longitudinal parity. If the
BCC is correct and no other receive errors have occurred, the 3964R procedure sends the
DLE character and returns to idle mode. If the BCC is faulty or a different receive error
occurs, the NAK character is sent to the communication partner. A repetition is then
expected.
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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