Siemens SIMATIC S7-400 CP 440 - Receiving Data with 3964(R); Figure 2-15 Data Traffic When Receiving with the 3964(R) Procedure

Basic Principles of Serial Data Transmission
2.5 Data Transmission with the 3964(R) Procedure
Point-to-point connection CP 440 Installation and Parameter Assignment
48 Manual, 09/2007, A5E00057742-03
2.5.4 Receiving Data with 3964(R)
Process of Data Transmission when Receiving
The figure below illustrates the transmission sequence when data is received with the
3964(R) procedure.
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Figure 2-15 Data Traffic when Receiving with the 3964(R) Procedure
Note
As soon as it is ready, the 3964(R) procedure sends a single NAK to the communication
partner to set the latter to idle.
Establishing a Receive Connection
In idle mode, when there is no send request to be processed, the procedure waits for the
communication partner to establish the connection.
If no empty receive buffer is available during a connection setup with STX, a wait time of
400 ms is started. If there is still no empty receive buffer after this time has elapsed, the
CP 440 reports the error (error message at the STATUS output of the FB) and the procedure
sends a NAK and returns to idle mode. Otherwise, the procedure sends a DLE and receives
the data.
If the idle procedure receives any control code except for STX or NAK, it waits for the
character delay time to expire, then sends the code NAK. The CP 440 reports the error to
the RECV_440 function block (STATUS output parameter).
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