Hi all,

1. i have s7315-pn/dp and two nodes on profibus communication.

strange thing happening all of a sudden im losing my communication to last node and its getting normalised on its own.

just for fraction of second which is captured by FB125 in ob86.

im pritty sure cables are fixed properly, please give me a suggestion.

2.in same configuration when the last node is powered off the remaining node is also not communicating, is the way it works.but if the 1st node is powered off still the second node is working normally???why?

Hello Bala;

Does the FB125 give you any indication of the cause of the lost communication? Is there anything in the diagnostic buffer of the CPU concerning loss of Profibus exchanges?

Your second problem could be linked to termination problems: if the last node has the termination resistior ON (on the Profibus connector) and you lose power, then the resistor is no longer powered, which leads to reflexions on the Profbus line. This can cause other nodes (or all of them) to lose communications.

See this FAQ for example (there are many more on the subject, if you search the forum):

Instructions for the permanent termination of a bus segment

https://support.industry.siemens.com/cs/fr/en/view/2157453

Hope this helps,

Daniel Chartier

Hi,

thank u so much for your reply..

1.im getting ob86 called frequently and fb125 im getting last node fault, and in Diagnostic buffer im getting only IO access error of the profibus node  input and output.

2.Yes i have terminated the resistor, is it must thar we should terminate the last node?

BALAKRISHNAN92

2.Yes i have terminated the resistor, is it must thar we should terminate the last node?

 hi BALAKRISHNAN92,

termination-resistors should be switched "on" on both ends of the Bus to avoid reflections.

regards,

Hi Balakrishnan92,

This can be fixed by use of an ACTIVE TERMINATOR. Plug active terminator at the end of profibus network, so even you power off  the last device on your node it doesn't make any reflections on the network.

Hi,

just for information:

FB125 was replaced to FB126:

Please check this link:

Diagnostic package PNIODiag for diagnostic evaluation for PROFIBUS DP and PROFINET IO (STEP 7 V5.x)

 Regards,

Towome