Backplane communication of a S7-300

[manolo40]

Hi all
I'm a french student and I have a research project which consists in detecting the late of the I/O cards and the internal bus of a S7-300 PLC.
I look for a documentation of the backplane (14 pins) bus but
Thanks for your help

 

[Peter Nachtwey]

There are two reason's for this.

1. PLC manufacturers don't normally want to make this information public. They don't want third party companies making boards for their PLCs. My company has made motion cards for a few different PLC manufacturers. We had to sign a non disclosure statement before getting the PLC bus specifications. The whole process is carefully controlled and any third party board must be certified by the PLC company.

2. Publishing information like this can only be used against the PLC company. If there is a good idea then others will copy it. If the bus design is poor then others will compare their better bus to the inferior bus. Usually it is the latter case that applies since there is only one bus that is best.

There are others on the forum that know more about the buses on a S7. They probably work for Siemens and even then they probably haven't seen the bus specifications. I have never seen specifications for a S7. I have talked to people that have told me what they think the S7 uses for its buses.

Why is it important? In the end it is a matter what PLC does the job.

 

[JesperMP]

Hi Manolo.

I dont think there is any published information about the backplane of the S7-300. Siemens do not want to let anyone know about these things.
What is known is that it is separated into the K-bus (for communication "Kommunikation") and P-bus (for i/o "Peripherie").

K-bus is the same as MPI. It operates normally at 187.5 kbps, but can be turned down to 19.2 kbps. The MPI/K-bus is shared with the MPI port on the front of the CPU. All specialty modules on the S7-300 backplane such as CP's and FM's get an MPI address. There is no galvanic isolation, so even if the MPI bus can be used for a a variety of operator panels, and PLC-to-PLC comms, it is actually not wise to make too big and complex systems with MPI. Everything is directly electrically connected together with no galvanic isolation.

Virtually nothing is known about the P-bus.
It is possibly derived from the backplane of the ancient S5-100U.
On www.sps-forum.de someone wrote that he had measured an approximate bus transfer speed of 5 Mbps by attaching a scope.
You may have to do your own research.

 

[JesperMP]

One important point:

I think that Siemens is ditching the MPI/K-bus part of the backplane.
A strong indicator that this may happen is that the new IPC in S7-300 format (S7-mEC) only supports the regular SM i/o modules. Not the FM or CP modules.
There is also no MPI or DP port on the CPU, only Ethernet ports.

I find it very good that MPI is dropped in favor of Ethernet.

 

[manolo40]

Thanks for your very fast answers.