Siemens PROFIBUS repeater 12 Meg problem
Introduction:
In 2025, Siemens announced that it would be retiring it’s PROFIBUS repeater (6ES7 972-0AA02-0XA0) in 2026. This repeater has been a workhorse of the industry for a long time. I was sad to hear that it was being retired.
This year, I found a problem with the old workhorse that I need to tell customers about. If you are running at 12 Meg, and you have a short in the cable on one segment, then this short will bring down the entire network which is not supposed to happen.
The Story:
In January 2026, I had a field service assignment involving a PROFIBUS network running at 12 Meg. There were five segments and most of the network was down. There were only a couple of devices responding. Nothing was in data exchange. The waveforms that we could see were awful.
My approach was to isolate each segment and verify communications. They were using Siemens repeaters, so I turned off the repeating action on each repeater and starting from the PLC, I verified each segment and moved along the network. I found the problem on the last segment and isolated the most likely cause. I replaced the cable and got the customer working. Everyone was happy, but afterwards, driving home, I thought to myself, why didn’t the repeater save the other segments. The short causing communications errors on segment five made sense to me. But why would it kill the other segments?
The Experiments:
This question kept nagging at me – Why would a short kill all of the segments? When I got some free time at home, I set up a testing area and started playing:
I set up the following test:
Notes on above drawing:
- SBA = Serial Bus Address
- The COMBRICK shown here is being used as an invisible gateway from PROFIBUS PA to DP
I placed the PBQ-One (a PROFIBUS bus monitor by Indu-Sol) on the controller side of the repeater. First, I ran the network at 1.5 Meg, and using an old Procentec PROFIBUS Fault generator, I put a partial short at the end of the second segment. The network continued working. All of the devices were in Data exchange.
I then tried it at 12 Meg, and control was lost. Serial bus address (SBA) 6 and 7 dropped off the network. The noise from SBA 4 on segment 2 was visible on segment 1.
I then tried the HMS ProfiHub B5, HMS COMBRICKs, Helmholz FLEXtra TwinRepeater (700-972-2AA02), and the Indu-Sol MULTIrep X2 (part number 110030010). Data exchange to all nodes were maintained when the fault was applied. The fault did not bring down the network like it did with the Siemens repeater. They all worked pretty much the same except for the MULTIrepX2 from Indu-Sol which had much higher signal levels than the rest.
Signal level of SBA 4 (where the fault was on segment 2) as viewed from segment 1:
Conclusion and Recommendations
Although the Siemens PROFIBUS repeater (6ES7 972-0AA02-0XA0) has had a long history and has worked well in most situations, it appears that at 12 Meg., it cannot handle shorts on cables.
If I was using Siemens repeaters on a system running at 1.5 Meg or slower, I would continue to use them.
If I was using Siemens repeaters on a system running at 12 Meg, I would switch out the repeater for a modern one. Based on my experience with the various repeaters, I would pick the MULTIrep X2 by Indu-Sol.