S7400/ET-200 PLC system, Communication problem

Even though PROFIBUS is relatively noise immune, poor earthing or very high noise levels (variable speed drives etc.) can cause significant errors. Check that the installation complies with PROFIBUS guidelines – it is a good starting point.

Above text is from a profibus guide. Almost allways the problem is bad earthing.

Syed Ziauddin, can you advise the following:-

1. Has this problem just started to occur - upon re-reading your post I would think that the system was functioning and then you encountered this issue.
2. Were there any recent modifications made to the system (Masters/Slaves added/removed)
3. Are you sure you have the correct GSD files loaded.

Regards

Mr. W.A Snow

Thanks for your interest,answers are:

1. This problem started since few days.

2. No modification being done recently.

3. GSD files are OK and sometimes system run for hours without any fault some time with continuous fault.

Best regards,

Syed Ziauddin

Syed Ziauddin, no problem. I trust that someday you will also assist me.

You might check out (alindsay81 also made reference to this):

http://www.automation.siemens.com/WW/forum/guests/PostShow.aspx?language=en&PostID=88172

Kind Regards

Mr. W.A Snow

I've seen problems like this a million times and its almost always a cable/earth problem

1. Have you changed the profibus plug on the last device, and the one before last?

2. How long is the profibus cable? you could install a repeater to give more length

3. what baud rate is your network running? if at 12mb try it at 1.5mb for a while and see if that helps

4. There is a Siemens device called a BT200 which can diagnose problems on profibus networks. get hold of one and see what it comes up with

5. What size is the 24vdc powersupply - it may be getting pulled down every now and again - install a 10amp supply and see what happens ?

Thanks,

Answers are,

1. We already have changed both plugs.

2.It is about 80mtrs. We are now asking Siemens to provide signal repeater.

4.We have heard about it ,but we will definitely try to get hold of it.

5.Power supply is PS407 on Master and PS307 on last and 2nd last slave.

We have many S7/400 ET 200 stations.

Whenever I have to troubleshoot problems like these, I have made up a Profibus test cable which I use to communicate to every ET 200 station directly.

In this way I can verify communication to the rack directly.

Of course you need a modified PLC simple program to do this testing.

I will highly recomment you do that . It comes in reallty handy when troubleshooting.