Suspect profibus problem - how do I proceed?

Hi guys,

I have a machine that has been acting up a few times lately. It looks like it "forgets" it's position.

The position comes from a profibus connected absolute encoder. The profibus cable is run in a cable chain where it goes a couple of feet back and forth, maybe three thousand times in a day, every day of the year.

The PLC is a Siemens CPU 315F-2 PN/DP and the encoder is the last device on the profibus.

• My wild guess is that there might be an intermittent problem with the encoder or the cable to the encoder. Is this likely?

• What happens if there is a problem with the cable and the plc and encoder doesn't communicate for a second or two? How is an error detected and what happens in the cpu? What would happen to encoder value in the plc during this break in communication?

• Also is the profibus cable even suitable to have in a cable chain for something that moves all the time? Or is this bad solution?

• How would you troubleshoot this problem?

Thanks in advance for any input or ideas you might have!

Best Regards,
Pete

Pete.S. said:

What happens if there is a problem with the cable and the plc and encoder doesn't communicate for a second or two? How is an error detected and what happens in the cpu?

Click to expand...

In a Siemens S7 PLC, an error-OB is triggered.
Investigate the diagnostics buffer.

Pete.S. said:

What would happen to encoder value in the plc during this break in communication?

Click to expand...

Depends on the setup of the encoder parameters. It may keep its last value. Or it may be set to zero.

Pete.S. said:

Also is the profibus cable even suitable to have in a cable chain for something that moves all the time? Or

Click to expand...

Plain Profibus cable no. But there are Profibus cables for every application. Investigate the brand and type of the cable.

Pete.S. said:

How would you troubleshoot this problem?

Click to expand...

Investigate the diagnostics buffer.
Look for cable or connector problems.
Checkup that there is a good equipotential earth.
Checkup that the screen is clamped to a earth, not only in the connector.
If nothing is revealed, try to exchange parts and see if the error disappears or moves.
If still nothing, consider renting or buying a profibus diagnostics tool.