What do extra terminations look like?

In both PROFIBUS DP and PA, having more than two terminations on a segment will lower the signal level. This lowers the overall robustness of the network and should be avoided.

In the case of PROFIBUS PA, the signal level is normally around 800 mV peak-to-peak.  Adding an additional termination will drop the signal level to around 600 mV, as shown below. This lower signal level can occur naturally if the network is very long. However, if the network is not very long and you see this, then you probably have an extra termination somewhere on the segment.

In PROFIBUS DP, the signal levels will vary from device to device and there can be multiple reasons for low signal levels; loose terminals, bad connectors or the device may just have low signal levels naturally. However, adding a termination will lower the signal levels for all devices on the segment. Below is an example where the regular signal level would be between +3 V and +4 V. Adding a third termination brings this down to close to +2V which is low.

 

How does this occur:

Anyone who has worked with PROFIBUS PA, knows that having more than two terminators is relatively common and certainly easy to do. Since many PA junction boxes have a termination switch or a place for a terminator, turning on more than the recommended two is easy.

PROFIBUS DP is a different story. When you turn on the terminator on the DB-9 connector, it will cut off the rest of the network. This makes it hard to have more than two terminations. However, just because something is hard has not stopped someone from doing it. The two ways that I have seen this done are:

  1.  A few devices have a termination switch on the PROFIBUS interface card. The customer may inadvertently turn on the termination switch on the card and on the DB-9 connector.
  2.  The DB-9 connectors are designed so that you extend the network by adding a wire to the ‘out’ of the connector. Some DB-9 connectors have a piggy pack connector that is supposed to be used only to connect diagnostic equipment. However, if you extend your network by plugging a second DB-9 connector onto the first one, then you have a setup where you can have double termination.

Conclusion:

Extra terminations lower the signal levels and this makes for a less robust system. Now, that you know how they can occur and what it looks like, you can avoid having this happen on your system.