Siemens SIMATIC NET PROFIBUS - Manual (Page 157)

Passive Components for RS-485 Networks
4-59
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
Note
The DIN rail must have a good conducting surface (for example tin plated).
Wall Mounting
Note
If the bus terminal is mounted on a wall, at least one PE terminal must be
connected to local ground. This connection should be over the shortest possible
distance.