Siemens S5 and S7 Communication problems


Thread Starter

Johan de Waal

Hi There,

We have 4 Siemens S5 PLCs and 10 S7 PLCs on a single PROFIBUS. We are experiencing major instability with communication between the S5s and the S7s. All the S7s seem to communicate without a problem, but the S5s lose connection to the S7s randomly. We installed repeaters on the bus, with no success. We changed all the settings there are available that might have an effect on the comms but cannot resolve the problem. (even running at the lowest baud rate possible). Once the S5 looses its comms all the S7s has to be disconnected from the bus, the S5s started up seperately and the one by one the S7 reconnected to the bus. Otherwise the S5s drop the comms....

Is this a known problem between these systems, or are we overlooking something?

Thanks for the help!


johan de Waal

Donald Pittendrigh

Hi Johan and others

Is this a DP or FMS connection/s.

Where in the world are you, I am in Richards Bay in South Africa, welcome to mail me direct if you need help in my S.A. at donaldp(at)

Donald P
I think it's almost accepted fact these days that 99.9% of communications problems on Profibus-DP are installation related. I don't mean that as a criticism, more just a statement of experience. Especially when erratic or unstable comms is the symptom. A mistake in the configuration or software will be most unlikely to work sometimes and not others.

Profibus is certainly a very robust and resilient network, but you do need to pay attention to the installation specs. Equipotential bonding (i.e. equal earth potentials for *all* nodes) is a must; correct termination at the appropriate points; good quality screening on the cable etc.

An experienced installer should be able to put an oscilloscope on the line, measure the waveform, and pinpoint very precisely if there are any trouble areas - probably to the specific slave or network segment causing the problem.
Hello Johan,

Do you take all the communication-partners in account? The S7 is much faster then the S5 and the "Profibus-profile" of Profibus must be taken over from the slowest {S5}!

Another thing is the triggering of the data, because the S5 are slower there might be a problem with the data troughput.

I think you have a timing problem because the S7 are faster with the timing on the Profibus and the S5 have to get the same timing.

If the Target Rotation Time is not the same for all PLCs then they will "fall off" the Profibus.

Best regards,

Mauricio Orellana

Hi I'm trying to configure a communication link between S5 and S7 with Profibus DP can you please tell me how did you do this? Card used etc...

Thanks in advence.

Mauricio Orellana

Dennis Pendleton


We are just about to replace an old texas plc and the same problem of Data transfer between S5 & S7 has been identified. Apparently Siemens sell a card (suprise suprise) that fits in the S5 rack to act as a data buffer / configurator. I am looking into the problem further, hence me being on this site, and hopefully can find a 3rd party bit of kit to interface between them until we cange over entirely to S7. Meantime I would appreciate you getting in touch if you find any further info.