Siemens SIMATIC CP 440 Manual page 36

Basic Principles of Serial Data Transmission
End Criterion: Fixed Frame Length
When data is received, the end of the message frame is detected when the
parameterized number of characters has arrived. The received data is accepted
from the CPU.
If the character delay time expires before the parameterized number of characters
has been reached, the receive operation is terminated. The character delay time is
used in this instance as the monitoring time. An error message is issued and the
message frame fragment is discarded.
Note the following if the message frame length of the received characters does
not match the parameterized fixed message frame length:
Message frame length of received characters greater than parameterized fixed
message frame length:
All characters received after the parameterized fixed message frame length is
reached:
– Are discarded if the monitoring time expires at the end of the message
frame.
– Are merged with the next message frame if a new message frame is
received before the monitoring time expires.
Message frame length of received characters less than parameterized fixed
message frame length:
The message frame:
– Is discarded if the monitoring time expires at the end of the message frame.
– Is merged with the next message frame if a new message frame is received
before the monitoring time expires.
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Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02