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
52 Manual, 09/2007, A5E00057742-03
Receive Buffer on CP 440
The CP 440 receive buffer accommodates 2000 bytes. During the parameter assignment,
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
parameter assignment interface or by calling the RES_RCV function block (see Chapter 6).
The receive buffer on the CP 440 is a ring buffer:
● If several message frames are entered in the receive buffer 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 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 those 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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