Siemens SIMATIC CP 440 Manual page 37

The figure below illustrates a receive operation with the end criterion "fixed
message frame length".
Figure 2-7
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Waiting for
character
Character
arrived
Character received
with length control
and character delay
time
Message
frame
complete
Message frame
entered in receive
buffer
Sequence of Receive Operation with End Criterion "Fixed Message Frame
Length"
Basic Principles of Serial Data Transmission
Error when receiving
Waiting for
parameterized
number of
characters
Expiration
of the
monitoring
Error entered in the
time
STATUS output of
the FB.
2-19