Siemens S7 Profibus random Node loss

Hello, I'm new to the forum so you my have to bear with me :site:

My site has two copper profibus networks joined by a plastic fibre. The main plc is at one end and we are currently seeing nodes being lost at the other end. The nodes disappear for approximately 200ms accordinging to the diagnostic buffer.

There are approx 10 nodes on the second copper network and all of the terminating resistors have been tested and appear ok.

We changed a 24v power supply that supplied all of the nodes that we had lost at some point, and this seemed to cure the issue as it went away for three months.

However, the fault has now returned and doesn't seem to have been caused by the psu.

I attended a Siemens training course last week and they said to check the earthing. This will be my next job but I wondered if anyone else had experienced anything similar?

Regards

Mark

Mark,

I had a similar problem with a Festo block on Profibus that was solved by grounding the 0V of the remote PSU (PSU in PLC panel was already grounded). So earthing may be the problem as Siemens have suggested.

My list will be:
* Grounding ov
* Screen of cable to ground EMC correct
* Distance from Profibus cable to motor cables and specially distance for cables for motors controlled by frequency inverter
* The terminating resistor - go for the active end termination ( https://eb.automation.siemens.com/mall/en/ww/catalog/products/10008830?tree=CatalogTree )

Do you have frequency inverter as Profibus nodes?? Some give problems om Profibus net at power off the inverter.

Check the fiber cable and OLM modules