Siemens Simatic S7-300 CP 341 User Manual page 70

Basic Principles of Serial Data Transmission
2.6 Data Transmission with the ASCII Driver
Receive buffer on CP 341
The CP 341 receive buffer accommodates 4096 bytes. During parameter assignment, you
can specify whether overwriting of data in the receive buffer is to be prevented. You can also
specify the range of values (1 to 250) for the number of buffered receive message frames.
The receive buffer on the CP 341 is a ring buffer:
● If multiple message frames are written to the CP 341 receive buffer: The CP 341 always
sends the oldest message frame to the CPU.
● If you want to transfer only the latest received frame to the CPU, you must assign the
value "1" for the number of buffered message frames and deactivate overwrite protection.
Note
If continuous reading of the receive data is interrupted for a certain time in the user program
and new receive data is requested, the CP 341 might first receive an old message frame
from the CPU and then the latest message frame. The old message frames are those that
were on the bus between the CP 341 and the CPU at the time of interruption or that had
already been received by the FB.
Extended error display at the receive FB
Activate the parameter "Display receive error at FB" to also display a message frame
received with errors at the status output of the function block P_RCV_RK.
If the parameter is deactivated, an entry is only created in the diagnostic buffer of the
CP 341.
70
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CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01