I am presently establishing a profibus network in my company. I am facing the problem of PLC going to STOP due to momentary rack loss fault in one of the remote I/O's. What are the possible reasons. there is no loose connection. Is there any chance of automatic warm restart by programming. What is the maximum distance between two profibus nodes. Ours is 150 meter

 

Hello;
First of all, we need to configure ther CPU to not go to STOP when it looses one of its nodes; if you are using a Siemens S7-300/400 master, insert a OB86 bloc in the project (no need to do any programming, just insert it in the program blocs and download it to the PLC. Not only will this allow you to continue if the event is present, but it will also enter more information in the diagnostics buffer on the PLC so you can identify the critical node. Unfortunately, you do not give us uch information on the type of PLC you are using; please give us more details.

I have worked with Profibus networks before, and I have found that one point to check when one has momentary node loss is the shield connection on the different Profibus connectors. You must make sure that not only the 2 wires (Red/Green, generally)are connected, but the shield must also make contact with the connector inlet (circuit ground); if that connection is not adequate, vibration may cause the wire to swing slightly, breaking the contact with the shield...and you have loss of the local node.

Hope this helps,
Daniel Chartier

 

Look at installing OB81 to OB 85. If You don't have one of these fault handlers in then the Proc will go to Stop. As for errors try slowing the profibus down to 1.5 Meg.

PLCLOGC

 

Load one of OB80..OB87 into PLC. I don't remember exactly which one, since I don't have Step7 on my PC now. You can load all of them or check which is necessary in Step7 help. Max. segment length for 12Mbit/s is 100m. For 150m max. transfer rate is 1.5Mbit/s.

I hope this helps.

 

Hi,

Just insert the OB86 (I/0 station failure) and the CPU will not go in stop. By using the parameters of this OB you can easyly create some alarms, statistics or what ever.

You can solve problems with your Profibus connection by slowing down the baudrate (12 MB/s => max. distance = 100 mtr., 187,5 kB/s => max. distance = 1000 mtr.) or increase the bus parameters (T[slot], retry limit, etc).

Hope that helps

Best regards,
Konni
[email protected]

 

Hello

i had the same problem with profibus using s7315-2dp with ET200S remote I/0 and festo CPA valves whit profibus node. the problem was that evry few minutes the plc fell to stop. i downladed OB82 & OB86 and then saw the problem in module infromation. my problem was a low voltage, i check it a solved the problem.

bye

[email protected]

 

From 3 to 12 Mbit/s, maximum segment length is 100m. If this is the set rate in yor case, you should decrease it.

 

Dear Daniel Chartier

Thank you for your reply. I am using CPU 315-2DP.

I have already loaded OB86 into the CPU. Still the problem exists. I am getting the profibus address of the node which is getting disconnected. But everything seems to be ok with connections and connecter. Will there be any performance degrading if i reduce the profibus transmission rate from 1.5 Mbps to 500Kbps. When I reduced the rate, the problem seems to be resolved. Pl comment

 

Hi T.N.,

I'm not understand if your profibus network is wired with optical fiber or copper.

If is wired with optical fiber I suggest you to check every inch of fiber, connector, junctions, etc.

If is wired with copper wires I suggest you to decrease speed, as Siemens suggest on 150 meter of bus, without repeater you can go at maximum 1 or 1,5 Mb/s, if you don't use Profibus cable (supplied by Siemens and other
manufactures) you must decrease speed very low due to maintain connection betweeen cpu and remote rack.

Best Regards

Gabriele Corrieri

 

Thanks for myour advice
It works with reduced speed at 500kbps

With Regards
T.N.

 

Hi, Check your net baudrate (as far as I remember for that length it shouldn't be faster than 1.5 Mbaud). Check if it's terminated properly. BTW, with S7 you can always avoid going to stop in such a situation by placing OB86 (even empty) into a program.

Regards,

Jacek Dobrowolski, M. Sc. E. Eng. Software Eng.

 

Hi List:

The maximum allowed baud rate on Profibus (copper) is dependent on the overall length of a Profibus Segment (that is the distance from the Master to the furthest Slave without repeaters), not the distance between two nodes. The maximum segment length is 100 meters at 12 megabaud. It is 200 meters at 1.5 megabaud and 400 meters at 500K baud. Up to 9 repeaters can be added to increase these lengths to a maximum of 1000 meters, 2000 meters and 4000 meters respectively. If your application can handle the decrease in performance, I would suggest decreasing the network baudrate to see if the problem goes away. You could also add a repeater before starting the 150 meter cable run to start a separate segment. One more thing to check is that the termination resistors are turned on at each end of the segment and that none are on inbetween. Note also that the overall Profibus network can only run as fast as the baud rate of the slowest device on the network.

To avoid having this problem halt your program you can also add the appropriate error OB (check CPU diagnostics) to your program. The error OB should contain program to take appropriate action in the event this faiure occurs without necessarily shutting down the entire processor.

Robert D. Wagner, P.E.

 

Hi T.N.
Some people here have suggest you to load OB86 for solve CPU-STOP condition problem, but from my advice, the very problem isn't stopping cpu (this is the consequence not the problem!) is the network fault. Reducing speed is a method for checking if problem is in wiring or in software: now your problem is in wiring: if 500 Kb/s get you a satisfactory rack scan-time you can leave 500Kb/s in network speed, but if you want to decrease scan time you must check wiring (cable and connectors). If you have checked all and problem isn't solved, and you want to go at 1,5Mb/s plan to use original Siemens connectors and cable, wire the bus cable in separate partition (shielded) from power cables and ground shield on every connector (both input and output cable).

Hope this helps you, but if you have some question mail to me directly:

Gabriele Corrieri
mailto:[email protected]