Why do we have to shield Profibus cables in both sides (at the beginning and at the end)?
In case Foundation Fieldbus we must shield only one side (at the beginning). What is the difference between both?

 

I'm assuming you are speaking of Foundation Fieldbus H1 and PROFIBUS-DP.

FF-H1 uses a physical layer which is galvanically isolated to 1000 V or more and totally floating. This physical layer is a true two-wire system. In my personal opinion the communication signal needs no grounding (in fact grounding the signal kills the communication). You only ground the shield - in one end.

PROFIBUS-DP uses RS-485 physical layer. Contrary to popular belief, RS-485 is NOT a two-wire system. For RS-485 you need a third path for signal return (pin 5) digital ground. This digital ground is usually the same as the instrument case/earth. Thus, all devices sharing the same bus are grounded through pin 5. If these devices are far apart (and with RS485 they can be) there will be ground potential differences and ground loop currents will appear, e.g. through the shield. For this reason it is important to establish an equipotential bonding scheme among devices to eliminate the ground potential difference and subsequent ground loops. This is why grounding at all points is required for DP.

Note that PROFIBUS-PA uses the same physical layer as FOUNDATION H1 so PA need not be grounded at every device, and shouldn't be. Just one ground point at the power source.

That is, PA and DP are different. Some may think that what applies to DP applies to PA as well, but electrically that is not the case.

Cheers, Jonas

 

Jonas:

I have investigated and I have reached the same conclusion. I have read some PROFIBUS manuals, and They said we only to grounded shield only both end.

Best Regards