Siemens Simatic S7-400 - System Modifications During Operation

R: Counter for retries
D: Default state
W: Waiting for character reception
Receive bu󹪜er on CP 440
The CP 440 receive bu󹪜er accommodates 2000 bytes. During the parameter assignment, you
can specify whether overwriting of data in the receive bu󹪜er should be prevented. You can also
specify the value range (1 to 10) for the number of bu󹪜ered receive message frames or use the
complete receive bu󹪜er.
You can delete the CP receive bu󹪜er at startup. The setting can be made either by using the
parameter assignment interface or by calling the RES_RCV function block (see Chapter 6).
The receive bu󹪜er on the CP 440 is a ring bu󹪜er:
• If several message frames are entered in the receive bu󹪜er of the CP 440, the following
applies: it is always the oldest one that is sent from the CP 440 to the CPU.
• If you only ever want to send the most recent message frame to the CPU, you must set the
value "1" for the number of bu󹪜ered message frames and deactivate the overwrite protection.
Note
If the constant reading out of the receive data in the user program is interrupted for a while,
you may 󹪝nd that when the receive data is requested again, the CPU 󹪝rst receives old message
frames from the CP 440 before it receives the most recent one. The old message frames are
those on their way when transmission between the CP 440 and the CPU was interrupted, or
which had already been received by the FB.
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
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