Siemens SIMATIC CP 440 Manual page 47

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 cleared
down, then an NAK is sent to the communication partner. A repetition is then
expected. If the block still cannot be received after the number of transmission
attempts specified in the static parameter record, 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).
Cleardown of a Connection for Receiving
If the 3964 procedure detects the string DLE ETX, it stops receiving and sends to
the communication partner a DLE if the block was received without errors. If the
block is damaged it sends an NAK. A repetition is then expected.
If the 3964(R) procedure detects the string DLE ETX BCC, it stops receiving and
compares the received BCC with the internally calculated longitudinal parity. 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 errored or a different receive
error occurs, an NAK is sent to the communication partner. A repetition is then
expected.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Basic Principles of Serial Data Transmission
2-29