Siemens Simatic S7-300 CP 341 - Sending Data with 3964(R); Figure 2-5 Data Traffic When Sending with the 3964(R) Procedure

Basic Principles of Serial Data Transmission
2.4 Data Transmission with the 3964(R) Procedure
CP 341 Point-to-Point Communication, Installation and Parameter Assignment
40 Manual, 09/2008, A5E02191071-01
2.4.4 Sending Data with 3964(R)
Sending data with 3964(R)
The figure below illustrates the transmission sequence when data is sent with the 3964(R)
procedure.
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Figure 2-5 Data traffic when sending with the 3964(R) procedure
Connection setup for sending
To set up the connection, the 3964(R) procedure sends the STX control character. If the
communication partner responds with the DLE character before the acknowledgment delay
time expires, the procedure switches to send mode.
If the communication partner answers with NAK or with any other character (except for DLE
or STX) or the acknowledgment delay time expires without a response, the procedure
repeats the connection setup. After the assigned number of unsuccessful setup attempts, the
procedure cancels the connection setup and sends the NAK character to the communication
partner. The CP 341 reports the error to the P_SND_RK function block (STATUS output
parameter).
Sending data
If a connection is successfully set up, the user data contained in the output buffer of the
communication processor is sent to the communication partner with the chosen transmission
parameters. The partner monitors the times between incoming characters. The interval
between two characters must not exceed the character delay time.
If the communication partner sends the NAK character during an active send operation, the
procedure cancels its transmission of the block and tries again as described above,
beginning with connection setup. If a different character is sent, the procedure first waits for
the character delay time to expire and then sends the NAK character to change the state of
the communication partner to idle. Then the procedure starts to send the data again with the
STX connection setup.
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