HI experts,

i have s7 317-2pn/dp communicating with other devices via profibus.

the plc is on run mode but the led bf1 is blinking red and sf is red.

I checked plc online, there are some error: I/O access error reading and I/O access error writing.

nodes 4, 34, 13, 52, 26, 23, 45 are not accessible via bus.

I checked the cables and i think it's OK.

when I disconnect profibus cable from A2-B2 the plc has access to all other nodes ( exept 52, 26, 32, 37 and 28 because the cable is disconnected.

but if I reconnect the cable again to A2-B2 and connect just the first device with adress 52, the plc can see the device but loses communication with 23 and 34.

I'm so confused.

what could be the cause of the problem?

waiting for your response please.

Any idea about the problem please?

Hello azelcemp;

You have 2 diagnostic LEDs active, BF and SF.

- BF (Bus Fault) will light up when you lose a station on the network.

- SF (System Fault) often appears in parallel with BF, when your program is truing to access (read or write) on a station that has been lost.

The fact that you re losing so many stations at one time, from all over the network would lead me to think you have lost power on one of your terminators.Loss of power on the termination resistors will cause reflections to appear on the bus, and the reaction of the network is very close to what you are seeing now.

See this entry from Procentec:

https://procentec.com/service-support/faq/profibus/

 Hello dchartier,

I checked all 4 nodes at the end of the network,

the termination resistor is activated in the 4 nodes, but the error persists.

dchartier

Hello azelcemp;

You have 2 diagnostic LEDs active, BF and SF.

- BF (Bus Fault) will light up when you lose a station on the network.

- SF (System Fault) often appears in parallel with BF, when your program is truing to access (read or write) on a station that has been lost.

The fact that you re losing so many stations at one time, from all over the network would lead me to think you have lost power on one of your terminators.Loss of power on the termination resistors will cause reflections to appear on the bus, and the reaction of the network is very close to what you are seeing now.

See this entry from Procentec:

https://procentec.com/service-support/faq/profibus/

Why is termination important?

Termination prevents reflections that can disturb the data communication. The higher the baudrate and the longer the cable, the more important termination becomes. Termination should be activated/placed at both ends of every bus segment. With PROFIBUS DP the termination is powered to provide an idle level when nobody is sending data.

Make sure that all the 4 end-nodes (where termination is active, nodes 2, 28, 21, 34) are always powered (they provide the required voltage to the terminating resistances n the Profbus connector). Check and if possible replace all 4 of these connectors, make sure their terminators are always ON. If for any reason (maintenance or replacement for example) one of these 4 stations needs to be removed from the network, place the resistor on the next station to ON before you unplug the end-node, or think of using an active Profibus terminator on the ends of the network (after the end station).

See chapter 4 of the following manual for more information on the active Profibus terminator:

SIMATIC NET PROFIBUS Network Manual System Manual

https://support.industry.siemens.com/cs/us/en/view/35222591

Hope this helps,

Daniel Chartier

Hello azelcemp;

The important point in my original reply is not that the terminators be active (set to ON) but that they may have lost power. Termination resistors must be powered at all times to prevent reflections on the bus, reflections destroy the telegram contents between nodes and swamp the Profibus master with error messages, something like you seem to be experiencing.

If you are confident that power was never removed from any of the end-nodes in your network, you should look into using a Profibus tester that will analyze the data exchanges over the network and report back on problems with cable lengths, connectors, termination, voltage levels, reflections, short circuits,  intermittent contacts...

You could look into Procentec's Profi Trace, or Softing's Profibus tester/analyzer. Or locate a local integrator who can provide such equipment and do the analysis at your plant.

https://procentec.com/products/profitrace/?content-1

https://industrial.softing.com/us/products/network-tester-and-analyzer/profibus-tester-5

Hope this helps,

Daniel Chartier 

Hello,

I have replaced a cable and some drives profibus cards in the segment wich genrates the errors then the network worked fine. 

thank you