When a Profibus DP device loses power, the communication to the rest of the DP devices attached on the bus system will fail.

True of False?

I assume the answer is false, since I know that the physical layer is RS485 which is usually configured as star, daisy chain, or T drops. Since Profibus is DP each device will have its own power source seperated from communications line, so that if one device power is turned off the bus still has a communications line connected.

Thank you.

 

However, if your device is the last in line and you do not have an active termination resistance (most devices have a switch to turn on the resistance if they are the last in line, however this only works if the device is powered as it is an active resistance) then chances are you will lose some or all devices on the bus due to reflection of the unterminated bus. This effect will be more pronounced on larger networks at higher speeds. So my answer to the question would be : false if the bus was correctly designed as per the recommendations of the profibus organization, true in some other cases.

 

If the Profibus device in question is the Master, then profibus communication to other devices will fail.

If the profibus device is question is the last node on the network and the termination point, then losing power will certainly reduce the voltage bias and if there are any weak points in the network then previously masked symptoms may appear and other node failures may start occurring.

We always bring the profibus network back to the PLC cabinet and use an Active Terminator.

Works for us.
Patrick