Siemens SIMATIC CP 440 Manual page 54

Basic Principles of Serial Data Transmission
Receive Buffer on the CP 440
The CP 440 receive buffer accommodates 2000 bytes. At parameterization you
can specify whether overwriting of data in the receive buffer should be prevented.
You can also specify the value range (1 to 10) for the number of buffered receive
message frames or use the complete receive buffer.
You can delete the CP receive buffer at startup. The setting can be made either by
using the parameterization interface or by calling the RES_RCV function block
(see Chapter 6).
The receive buffer on the CP 440 is a ring buffer:
If two or more message frames are entered in the receive buffer of the CP 440,
the rule is that the oldest message frame is always transferred by the CP 440 to
the CPU.
If you only ever want to send the most recent message frame to the CPU, you
must parameterize the value "1" for the number of buffered 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 find that when the receive data is requested again, the CPU first
receives old message frames from the CP 440 before it receives the most recent
one.
The old message frames are the ones that were on their way when transmission
between the CP 440 and the CPU was interrupted, or which had already been
received by the FB.
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Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02