Profibus DP problems with S7 1

[Oxidation]

I'm working on a system, remote I/O station, with Profibus link to an S7 & a Intellution package to show the process. The Profibus link is around 200m long & I'm getting regular "ghost" trips on the process plant due to momentarily lost comms or several transmitters suddenly show alarm levels. I've noticed that the shield of the Profibus cable is earthed at both ends!? The supplier of the system informs me that this is because Siemens actually recommend it to be so?!
Has anyone had any similar experience & how did you fix it?

 

[controlsdude]

Oxi

Check the following:
make sure that the Mbit rate is capable of the distance. I believe I always used 1.5Mbaud = 200 meters, this is a guess since its been awhile, but check the specs online for distance and baud rate.

Also, if the distance is problem, get a repeater installed at the problem area to extend the distance.

220 ohm resistor termination on both ends, usually a slide switch, check resistance, make sure your network is powered down to perform this test. If you turn on slide switch on one end and turn off on the other do a resistance check on the non terminated end and you should see 220 ohms, that way you know that the cable is good.

If you have slide switches on all connectors, shorten the network in half and see if all is well, then keep adding more cable by taking out termination and putting the termination in at a greater distance.

most profibus connectors from siemens are idiot proof, but check the slide switch and check a few for that the vampire connector if used is stabbed correctly. I had bad big screw driver electricians who did not do a good install. Green wire goes to green wire connector, red wire goes to red wire connector, shield is passed by the connector also by its connection.

check the dip switch settings for proper address settings.

Most profibus network configuration have good tools with them, and siemens is no exception. Use the tools that they give you and use the procedures above and you should get to the source of the problem.

 

[krkautomation]

All good advice from jmounce (a '*' for that).

If this problem is due to noise rather than a bad installation, you might want to consider changing the 200m cable section to fibre optic...

 

[Oxidation]

Cable has been checked before I posted here, the resistors are fine & the baud is as per documentation.
However, the reason for grounding the Profibus cable shield at both ends of the installation is the answer I would most like to clarify.

Thanks

 

[controlsdude]

Oxi

I believe Siemens said the same thing to me on an install, I would follow their advise. Is their anyway to call your local rep and get ahold of a siemens profibus expert? This sounds like something that is particular to their profibus networks and the germans are particular when it comes to doing things. German way or the highway. I believe as long as the shield is passed with the connector you should be ok. If their is a break in the shield along the cable run then it could pose a problem.

 

[krkautomation]

Hi Oxi,

Over the years I've encountered various opinions on the 'best' methods of earthing cable shields for different types of installation etc and I've come to the conclusion that it will always be a point of contention in the electrical engineering community...

Anyway, for some light reading...

The Siemens PROFIBUS network installation manual is here...

http://support.automation.siemens.com/WW/view/en/1971286

See pages 270 - 271


and some other information that you may find informative...

http://forums.mrplc.com/index.php?download=502

See pages 106 - 107

 

[ozmosis]

Yes, the shield should continually be bonded at each node. Your Profibus connections will (should have) provision to terminate the shield effectively as they come in and go out of each device.
Terminating the shield is one thing, the others involve addressing the grounding and effective bonding of your respective devices to ensure a common ground plane and ensure you have equipotential bonding otherwise you can introduce other problems that could be seen on the profibus telegrams.
As already mentioned by jmounce, check your baud rate. Profibus is capable of 12Mb but the 200M and introduced noise if you bonding is not effective will limit this.
There's some useful reading on the end of this link:

http://www.sea.siemens.com/automat/product/net/aulit.html

 

[djs]

Hello;
The Profibus Trade Organization recommends grounding the shield at both ends. I know that this goes contrary to what is done with 4-20ma signals.
It really sounds like you are having noise problems. It there is to much noise, then the I/O drops stop data exchange with the master (S& PLC) and wait to be re-configured from the master. This will most likely result in various alarms being generated. Try the web site

http://www.profibus.com

for some installation and trouble shooting help.
If you still can not find the problem then you will need to find someone with Profibus diagnostic tools.