Siemens SIMATIC CP 440, Руководство пользователя

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Basic Principles of Serial Data Transmission
2-15
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
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”.
Waiting for
character
Character
arrived
Error when receiving (not a
character delay time error)
Message frame buf
fered
End criterion for message frame identified, error entry follows
Character received
with character
delay time
monitoring
Error entered in the
STATUS output of
the FB.
Waiting for character
delay time. Received
characters discarded
Message frame
complete (character
delay time expired)
Message frame
entered in receive
buffer
Figure 2-5 Sequence of Receive Operation with End Criterion “Expiration of Character
Delay Time”