Siemens SIMATIC CP 440 Manual page 59

Example 3: Neither correct polarity nor correct default settings known
– Reverse the connections (note: cross both pairs of lines in the RS422)
– If not OK, change the default settings (all options) with an appropriate
communication attempt
– If not OK, change the connections back, and change the default settings (all
options)
– Check the security word (e.g. CRC)
When setting up again, don't forget to put back the terminating resistors you
removed.
5. Additional tips:
– If possible, connect an interface tester (possibly the V.24 converter
RS22/485) to the connecting cable.
– Check the signal level using a measuring device (measure level to GND
(pin 8)).
– Some devices do not signal reception if data is being received but the CRC
security word is not correct.
– If necessary, replace the module to exclude the possibility of an electrical
defect.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Commissioning the CP 440
3-5