Siemens S7CPU can't see slave DP on Profibus


Thread Starter


I have a problem, I have a S7 PLC, 416-2 Dp. it has 10 slaves on the DP network. The machine was running and then shipped overseas. now the S7 has a bus and system fault and none of the slaves can be seen online; they all "module not available". Since the PLC sw was not changed, I initially thought that I had a cabling problem. I checked all and even added new profibus cables. I modified the HW config, to only show the 1st rack and put just a single cable on that rack only, still have the same problem.

If I go into the HW config, and to the DP and "disconnect the master system" compile and download, the bus and system faults go away.
Any ideas?

Daniel Chartier

Hello Mike;

One thing that could easily cause this type of problem: did someone do a download to the PLC from a programming computer where the gsd files might be missing? Missing gsd files would stop he PLC master from "seeing" the slaves on the bus, as the gsd files provide the communication interface the master will use to exchange data with the slave.

One way to find out if this is a problem is to open the Profibus configurator (in HWConfig) and look at the icons used by the system for the slaves. Normally they should have a specific icon for each. If the are greyed out or have only a "DP-Norm" icon it means the gsd files have not been integrated oin the local computer.
If you go Online while the HWConfig is open you will also have access to the local diagnostic buffer for all the slaves (especially those marked as "defective"). This could give you more indications on the cause of the error.

Hope this helps,
Daniel Chartier

Gabriele Corrieri

I've this mail on desk from two days, the only answer that I could give is that the DP port is defective: DP port are generally well protected against external overvoltage, but some works such as welding, or earth connection very weak can do a port damage: I suggest to you two troubleshooting ways: with a 2 channel scope, groung on pin 5, ch1 on pin 8, ch2 on pin 3 you must see a square waveforms on both channels.Second ways of troubleshooting: your CPU has 2 port configurable as DP o MPI: try to move the entire master DP on other port.

I've voluntarily not suggest to check termination on both ends, that termination ends are powered by slaves (by 5V), that the cable is right (especially when DP speed is over 1,5Mb/s): you could (when you're sure that the DP port is electrically running decrease the DP speed temporarily to 187,5Kb/s or much more less to exclude defective wiring, termination or connector)


Gabriele Corrieri
[email protected]