Siemens SIMATIC CP 440 Manual page 34

Basic Principles of Serial Data Transmission
End Criterion: End-of-Text Character
When data is received, the end of the message frame is detected when the
parameterized end-of-text character(s) arrive. The following options are available:
One end-of-text character
Two end-of-text characters, 1st and 2nd end-of-text characters
Two end-of-text characters, 1st or 2nd end-of-text character
The received data including the end-of-text character(s) is accepted from the CPU.
If the end-of-text character is missing in the received data, the character delay time
elapses during reception and results in a termination of the frame. 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.
If you are working with end-of-text characters, transmission is not
code-transparent, and you must make sure that the end code(s) is/are not included
in the user data of the user.
Note the following when the last character in the received message frame is not
the end-of-text character.
End-of-text character elsewhere in the message frame:
All characters including the end-of-text character are entered in the receive DB.
The characters following the end-of-text character:
– 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.
End-of-text character not included in message frame:
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