documentation on using a dedicated RS485 module

docs/FAQ/RS485_tips_and_tricks.md

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 Consider [this article on electrical-engineering](https://electrical-engineering-portal.com/correct-cabling-modbus-rs485) a must read. 
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+
+### **Using a dedicated Wago RS485 module**
+Depending on requirements a dedicated RS485 module might be required instead of using the onboard serial port of the device (if available).
+
+Based on user experience in this field, pay attention to:
+- reset the module to factory settings before using.
+- last confirmed working version of required libraries:
+    - WagoSysModule_75x_65x: Version 1.9.5.2 
+    - WagoTypesCom: Version 1.6.1.3
+- change the 'comport' in the RS485 master function block you intend to use.
+- make sure the wiring of the module is correct:
+
+<img src="../_img/WagoRS485ExternalWiring.gif" width="550">