Siemens SIMATIC NET PROFIBUS Owner's Manual page 162

Passive Components for RS-485 Networks
Keep to the permitted ambient conditions
Remember that a standard connecting cable cannot stand up to the same
environmental conditions as an uninterrupted LAN cable. If necessary, provide
extra protection for the connection to avoid dampness, dust or aggressive gases
causing problems by covering the connection in a cable sleeve. You can find
information about ordering this in Appendix I-2 "SIMATIC NET Support and
Training".
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PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000