Siemens Simatic S7-400 - Page 51

4. Check:
Always monitor the TXD send and RXD receive LEDs during the following operations
Example 1: The polarity of the line is de󹪝nitely correct
– The default settings (all options) vary
– Check the security word (e.g. CRC)
Example 2: The default settings are de󹪝nitely correct
– Reverse the connections (note: cross both pairs of lines in the RS422)
– Check the security word (e.g. CRC)
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 → RS 422/485 converter) to the
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.
Commissioning the CP 440
Point-to-point connection CP 440 Installation and Parameter Assignment
Equipment Manual, 03/2022, A5E00057742-AE 51