Siemens S7 300 PLC with profibus

In plant we use Siemens S7 PLC CPU 319F-3 PN/DP with profibus connected to remote I/Os. Profibus network have profibus nodes. We are getting frequent profibus communication failure. Specially during start up of plant after shutdown. We replaced profibus cables/connectors. But still it is happeneing each time.

Could anybody please give some suggestion/advise or document how to proceed to rectify the problem. your reply will be great help.

Here's what comes to my mind while drinking coffe on a Sunday morning .

1: Check the plug connections; screen properly at the gland plate. All plugs with only 1 cable must have the terminating switch ON.

2: Check the profi network with an analyzer. The ProfiTrace from www.procentec.com is popular.

3: Check the ground connections between cabinets/nodes.

4: The Profi cable is not allowed to be laid close to power cables due to EMC. You can find more about trouble shooting in the handbooks here: www.profibus.com. VFDs and thyristor power units are the worst noise sources.

5: Quick (and dirty?) trick: Change bus profile from "DP" to "User defined". Increase the "Retry limit" to f.x. 5. NB! Click on the calculate button to see if the new times are acceptable for you. Compile and download.

6: You'll find a lot more tips if you search the forum.

Kalle

BTW, here is a link to a very compact and informative EMC do's and dont's handbook from IFM. The contents should have been etched into the mind of all el/aut folks.

http://www.ifm.com/obj/EMC-Pocket-Guide-ZVEI-english.pdf

Kalle

!!Grounding/shielding is very important!!
Everywhere, where grounding is possible, that should be done.
When I have measured lines, most of problems caused by insufficient groundings.
When peeling cable, aluminium foil between wires and metal shield should not be turned over metal shield, because there is insulation layer at the metal foil. Metal shield should have straight contact to connector grounding.
Also cable type is important. Cable has to be profibus cable.

Profibus speed could be also decreased, if there is no "need for speed"..
If speed is "default" 1.5, and threre are lots of connections, stub lines and long cables, speed could be decreased.

All cables should be AT LEAST one meter long, specially if speed is 1.5mbps or more.

Hope these helps..

I am at my forth plant straightening out their wiring errors. Most of the time the Profibus cable is too close to the power cables. Look in the MCC buckets and wiring risers on the sides of the buckets. Then look at the cable tray. If any cables are within 4 inches of the Profibus cable, move it. Be especially careful around VFD's. They will screw you up quicker than anything. Usually the symptoms are that you have a motor running and you start something else and the motor you are running stops.