Siemens SIMATIC CP 440 Manual page 101

Time Sequence Chart for the RECV_440 FB
The figure below illustrates the behavior of the NDR, LEN and ERROR
parameters, depending on how the EN_R and R inputs are wired.
EN_R
R
NDR
ERROR
LEN
(length)
Figure 6-2
Note
The EN_R input must be set statically to "1". During the entire receive job, the
EN_R parameter must be supplied with RLO "1" (result of logic operation).
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Time Sequence Chart for the 9 RECV_440 FB
Communication Using Function Blocks
6-13