Siemens SIMATIC CP 340 User Manual page 52

Basic Principles of Serial Data Transmission
2.5 Data transfer using the ASCII driver
End Criterion "Expiration of Character Delay Time"
When data is received, the end of the message frame is recognized when the character
delay time expires. The received data is taken over by the CPU with the function block
P_RCV.
In this case the character delay time must be set such that it easily expires between two
consecutive message frames. But it should be long enough so that the end of the message
frame is not falsely identified whenever the partner in the link 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 2-15
52
Sequence of Receive Operation with End Criterion "Expiration of Character Delay Time"
PtP coupling and configuration of CP 340
Manual, 04/2011, A5E00369892-03