Siemens SIMATIC CP 440 Manual page 33

End Criterion: "Expiration of Character Delay Time"
When data is received, the end of the message frame is detected when the
character delay time expires. The received data is accepted from the CPU.
In this case, the character delay time must be set such that it reliably expires
between two consecutive message frames. However, it should be long enough so
that the end of the message frame is not falsely identified whenever the
communication partner takes a send pause within a message frame.
The figure below illustrates a receive operation with the end criterion "expiration of
character delay time".
Figure 2-5
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Waiting for
character
Character
arrived
Character received
with character
delay time
monitoring
Message frame
complete (character
delay time expired)
Message frame
entered in receive
buffer
Sequence of Receive Operation with End Criterion "Expiration of Character
Delay Time"
Basic Principles of Serial Data Transmission
Error when receiving (not a
character delay time error)
Waiting for character
delay time. Received
characters discarded
Error entered in the
STATUS output of
the FB.
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