Have you ever wondered what would happen if you took the cable at the end of a PROFIBUS network and connected it back to the beginning of the network? Well, I have. Being involved with Troubleshooting, I have thought of all the different ways you could mess up a network. I even tried this with my training racks. The results were a little boring. Every device went into a bus fault, and you could not see anything with a bus monitor. However, that was a short network.

On a field service call, I recently found a PROFIBUS network that WAS looped back to the start, forming a ring. IT WAS WORKING!!!! Not very well, mind you, but it was working. I was amazed. Below is the waveform.

This is something that should not work at all. My guess is that the network was long enough to get away with it. When I first saw this waveform, I thought that there were two devices at the same address. It will produce a somewhat similar waveform (see my post “Using PROFIBUS DP waveforms to troubleshoot network issues” for more details). However, that was not the case. They plugged a connector with two PROFIBUS cables into a piggy-back connector with only one cable wired in.

Unintended PROFIBUS loop

Unintended network loop

These piggy-back connectors have the connector on the outside to provide an access port for diagnostic equipment. You must never extend the bus by plugging another cable into it.

PROFIBUS is a serial protocol that is wired in a daisy-chain manner from start to end in a line configuration. We sell special ‘repeaters’ called ProfiHubs that have software in them that will let these repeaters be connected in a ring topology.

In this case, the customer was not trying to create a ring. I believe that this happened by accident. It looks like they had extended the network through this cabinet by using a DB-9 connector to join two sections of cable. Then, sometime later, someone was probably troubleshooting another issue. They saw the cable hanging there and decided to plug it in.

The solution to prevent this from happening is an easy one – get your people trained. The rules for wiring PROFIBUS are pretty easy, but if you do not know them, stuff like this can happen easily. JCOM Automation offers three levels of PROFIBUS training. Please see our Training page for more details.


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