Profibus connection fault

Hello,

I have problem with PROFIBUS DP communication. My knowlege about profibas communication is on basic level. 

I have SCADA PC connected with 4 PLCs throught profibus. The communication was good for years, and one day it just started to fail between two PLCs (PLC 3 i PLC 4 - that have the longest distance from SCADA PC).

First i lost connection with PLC - 3, which is the farthest (about 200m cable from SCADA PC).

Then fault changed to PLC - 4 (140m distance cable), and PLC - 3 was fine in that moment. Then in couple of weeks fault was started to change between this 2 PLCs (3 and 4) alternatley - one works, other is in fault.

First connection of profibus is on SCADA PC, and terminator on connector is set to ON. Other wiring of connection is little bit wierd becouse i have all terminator on the end of line set to OFF (and its hard to see which one is last). I tried to put terminator to ON on PLC 3, and then i lost connection on PLC4 and even on PLC 2. But when i tried to put terminator ON on PLC 4, i lost connection on PLC 3.

I dont know what to do more, all was set like it was when all was working fine, but something cousing fault between this two PLCs.

Hope you guys can suggest me what to do.

The wiring is executed exactly like it is shown on picture.

BR

THOMPSONvk

What is this connection box in your drawing?

Always when a profibus cable ends to a device, the termination should be ON. In your case, all the PLC's that are connected to the connection box.

Hello Thompson;

Hati's answer is spot on.

Profibus-DP normally uses daisy-chain connectivity, as shown in the very primitive drawing below. Whenever you introduce a T-junction (like your connectivity box) to design a star topology, you must considered the T-junction as the end of a segment, each branch as a start of a new segment, ant the terminators must be active entering and exiting the connectivity box.

At lower transmission speeds and small lengths of bus, RS-485 (Profibus--DP, in this case) can be tolerant of some termination issues, which is probably why this installation worked without problems for some time. However, the first time a termination issue appeared, you lost the consistency of the network and you find out that your topology is not as robust as you thought.Now termination issues appear and you lose different stations in different branches.

So, as Hati requested earlier, what exactly did you use for the connectivity box? Could you consider changing it for a RS-485 repeater, with which you could establish your star network, or redoing the Profibus cabling to return to the standard daisy-chain structure?

RS-485 repeater manual:

https://support.industry.siemens.com/cs/ww/en/view/48598071

Hope thia helps,

Daniel Chartier

Thank you both for your suggestions, Hati and Daniel.

I get network to work good now. I didnt do nothing much. I only replaced one connector on PLC - 3, and after while it started to work all. Hope, it will stay stable. 

All connectors at the end of connection box are set to OFF (PLC - 2, PLC - 3, PLC -4).

I m sending you picture of 'connection box'.  

I've learned about that repeater and how to connect PROFIBUS DP network well. Thank you for your information.

BR

THOMPSONvk

Hello Chartier

After a couple of months my network started to have problems again.

I eventually remodeled my profibus network like its suppose to be (like scheme on your drawing), without star connections and without repeater.

My network is working fine now.

My last PLC of the network can't be terminated on connector (ON state), becouse when i put connector to ON i lose all connections on my network. So i put it to OFF.

Thank you again for all your suggestions on this post.

THOMPSONvk