S7 318-2dp Profibus Clearing Program

Has anyone had a problem with a profibus short clearing proccessor?

We have talked to siemens tech support they said it was coming in on the shield of the profibus wire and suggested to remove the ground shield at the proccessor. we did this and had another short it did the same thing.

What's the evidence that the source of this fault is from Profibus? Obviously the Siemens' tech support advice hasn't solved anything, but if the problem wasn't from that source to begin with then it's hardly surprising. When you refer to removing the ground shield, is this disconnecting the braided or foil shield? Personally I wouldn't recommend doing this in a Profibus installation. The standard Profibus Organisation installation guidelines expect 3 connections (2 signals and ground) and I would make sure there are 3 connections.

Which Siemens PLC is this? If it's S7-300/400, then even if the memory is scrambled I've been assured there are two things that can never be lost: the diagnostic buffer, and the MPI address. What does the buffer contain?

Has the system just recently produced these symptoms, and if so what else in the plant/factory/town/city etc has changed to cause them? If it's always had these problems, we need to analyse when and how they occur, and what else happens just before or at the same time.

More info needed ...

Regards

Ken

Ken - It does say 318-2DP in the title.

Rob2 - What is being shorted to what ?

Ken it is a S7300 As far as the buffer there is nothing in it.
all lights are flashing rapidly on the processor when you turn the switch from run to stop and back to run you have no bus faults no system faults almost like you did a reset on processor.
As far as knowing what caused this problem once the cable got melted.One time we had an end resistor get soaked in water.

Thanks Robbie

"...once the cable got melted.One time we had an end resistor get soaked in water."

Are these two separate occasions? What on Earth are you doing to this poor network? Setting it on fire and then sticking it in a bucket of water to put it out? I would seriously consider a full cable and plugs re-installation. Who knows what damage has been caused to conductors and insulation with these extremes. Water intrusion and the effects of corrosion etc are persistent and creeping - you'll never be sure you've got rid of it otherwise.

And if the buffer's empty I guess my info was wrong - it looks like things can erase that. (I thought even the most complete reset didn't empty the buffer). Let's hope I'm right about the MPI address otherwise we're all stuffed!

Regards

Ken

all lights are flashing rapidly on the processor

Click to expand...

Ive had the flashing LED thing happen to me,it was due to a configuration Error on a MPI Network.

It didnt wipe the program, however i couldn't access the diagnostic buffer in this condition. The magic cycling of power was the only way to clear the LED's and re-establish a connection, unfortunately the diagnostic buffer was empty also.

With respect to the diagnostic buffer, i was told on the serious faults you get a "no relevence to end user" message and a code for siemens personnel.

Dont know how true this is? not very helpful though!

Rob2 said:

As far as knowing what caused this problem once the cable got melted.One time we had an end resistor get soaked in water.

Click to expand...

You are not telling us much here, but I will take a jump and guess that it is the MPI cable that got shorted. Any electrical problem on the MPI bus can zap the processor as the MPI port is not isolated from the CPU or backplane. The Profibus port do have isolation.
An electrical short should normally not happen, and neither should a connector be dipped in water. But things happen, that is why (non-isolated) MPI should not be used for things like outdoor, often changing installations, or mixed control panels.
You can add a repeater to achieve electrical isolation.

Also, it is perfectly acceptable to remove the shield in the connector, but then you must tie the shield to a ground bar in the same panel. This is actually a better method than terminating the shield in the connector.

I've also seen those symptoms (flashing lights, complete loss of program, etc) and it was caused by a modem connected to the MPI port. Apparently there was noise or a spike coming in on the line sporadically. I removed the modems four years ago and have had no problems since then.

I suppose the same thing could happen on the ProfiBus network, but I'm not sure. You need to look at your installation to prevent the type of damage you describe.

I have seen some wierd stuff happen. I would STRONGLY recommend putting a fiber link in somewhere to protect the PLC. Many times people will have different protentials to ground and blow the hell out of stuff. The componets only run on 5 vdc. It don't take much. If you are running between buildings with copper, you should be shot. (just kidding) Just in case you never heard, electricity and water, don't mix. Neither does fire. Fiber opics is your friend!

The Diagnostic buffer When this happens
Has Defective: internal system error

memory reset started automaticaly {power on not back

Rob2.

What I dont get is that you seem to have frequent problems like shorting or getting the connector dipped in water.
Strange. Your installation should try to prevent this from happening. In "normal" cases such errors should be very rare.

If you cant prevent it from happening, then try to add a repeater.

This dont happen very often and we do have a repeater

You have a repeater, but still the CPU program gets cleared !
That is a bit shocking actually.

Can you inform us more details.
Where does the MPI or Profibus cable go to ? Outdoors ? Other panels ? Or ?
What is the exact layout of the Profibus or MPI network ?
What are the distances, what components are used ?

Also, is the backup battery OK ?
Is the program stored on the Memory Card ?
Is the ground OK ?

Picking up on two points from previous posts -
RRobbins' "Many times people will have different potentials to ground ..." and Jesper's "Is the ground OK ?"

I think this is by far the most common area of problem I've encountered in 'susepct' Profibus installations. A well installed network is very resilient, but two things are essential. Each node should be earthed (grounded); and each node should be connected to an earth (ground) at the same potential. Profibus standards are absolutely clear about that last part. Even within a single building, I have heard of multiple grounds with different potential. Fibre-optic certainly resolves any electrical transmission problems, but brings with it a new set of issues in terms of components, costs, re-cabling, skills required for installation etc.

Regards

Ken

Correction !

Previously I wrote:

Also, it is perfectly acceptable to remove the shield in the connector, but then you must tie the shield to a ground bar in the same panel. This is actually a better method than terminating the shield in the connector.

Click to expand...

This is wrong. Upon checking I find that you MUST connect the shield at the port of the connected device. You can then improve the ground connection by clamping the shield at a dedicated grounding bar, but the shield at the connected device must not be removed.

If there is a problem with large distances and/or high potential differences, then equipotential grounding shall be provided as a minimal measure.