We have a profibus DP system with a master (416 2XK01-0AB0) and different slaves, among which there are 2 153 racks, a S7200 PLC and a S7315-2DP PLC.

It has been running for years. Suddenly it begins giving communication problems, one of the 153-racks looses the connection every 10 seconds or so.

After having changed cables, revised connectors etc it was seen (with a Profibus analyser) that the 315-2DP slave was changing between master and slave status (which as far as I know isn't possible). We saw as well that the 315 was scanning the network with FDL-telegrams.

A rather strange thing. Then we removed the setting "Programming, status/modify or other programming device functions and unconfigured communication connections possible" on the 315 "Operating mode" settings. And the problem was solved.

I am not quite sure what this setting means. Is there any good explanation?

Hello Moreno;

The Test/modify setting allows you to program the slave CPU over Profibus, as if it was a MPI node. It also allows your programming software to execute test functions to determine the state of the Profibus slave node.

See this page from the S7-300 CPUs Operating Instructions manual:

https://support.industry.siemens.com/cs/us/en/view/13008499/28860004747

The problems you faced might have been due to the extension of the Profibus cycle that this setting causes (see the screenshot below). An older electronic equipment sometimes develops strange and unexpected quirks...

Hope this helps,

Daniel Chartier