Sinumerik Alarm 120201 (Communication interrupted)

[AlfredoQuintero]

Hello:
I am at a customer's factory in which a 20+ year machine from Germany is occasionally showing Alarm 120201. I was asked to diagnose a Profibus DP line, but it turns out it is an MPI line. I am diagnosing with Softing's BC-700 and the tool shows good physical layer and communication parameters. The baud rate is 1.5MBaud and there are only 4 stations communicating through MPI.
everything looks normal. Scope traces look acceptable and the total bus length is less than 5 meters. Using Siemens Profibus connectors with proper termination. The cable is a purple cable and feels a little bit soft. Checked one connector and was made with ferrules. Assembly looks OK.
I have never seen a Sinumerik line before. mostly I had done Profibus DP diagnostics so far.
What can cause this alarm 120201. Unfortunately the HMI does not say what is the node in the MPI line that stops communicating. I see four nodes and most of them are broadcasting data to station 127.
Anyways, if someone has experience with this alarm, will be glad for your comments. As per the customer explanation, it clears automatically. For some reason they say it yesterday many times. I have been here for 3 hours and have yet to see this.

 

[AlfredoQuintero]

Maybe I did not provide enough information?
Here is some of the data I was able to collect. Customer has asked me to show-up tomorrow afternoon again, because he saw the problem yesterday (when I was not there, of course).
Let's hope the problem is seen while I am there again. The Sinumerik is connected to the MPI bus on connector X101.

 

[AlfredoQuintero]

A quick update. I went to the customer's shop floor again and did further test. One hint came when I disconnected the MPI board for the hand-held device (MPI station 15), which is one of the terminal stations in the bus, while we were measuring the quality Index. I did this by setting the resistor to ON position on the previous Profibus connector (on station 6). The quality index shown in the Softing tool increases dramatically, showing that there was something wrong with station 15.
We did the cable test starting from the station 15 side. Cable test step 1 without any connection passes, the cable test step 2 which is test of the farthest terminator (the Sinumerik station 2) pass, but then the third step of the test (step 3 or test of terminator at measuring station), that is station 15, fails. If the third step is measured on the next station (station 6) the test passes. When measuring voltage of termination at station 5 the voltage is zero. Something is not good with this MPI boards termination circuit.

With the station 15, (the hand-held MPI board as I said) connected to the bus, the measured idle voltage is 600 mV, which is not good.
What I did is I connected a Profibus repeater configured in such a way that one one side we have only the hand-held MPI board terminating the Profibus connector and also terminating the repeater on this side of the repeater (in our case segment 1 of the repeater). On the other side of the repeater we connect the other three stations (the PC station 1, the Sinumerik station 2 and the other MPI station 6). Quality index shown in this configuration improves significantly from the network without repeater, and the idle voltage is 1.2 V, which good.
Next step is to get a new MPI board for the hand-held device, but I am not sure if this is even available as this machine is over 20 years old, but at least for now the system seems to be running as expected with the repeater.
I am taking the time to document this issue because it looks to me as though there may be similar machines all over the world, so if this information is helpful to some colleague in a different corner of our planet, I would be happy to hear back.