I have a project involving a couple of oems each designing a part of the system. I would like to assign each vendor a range of MPI and DP addresses for their piece. This will ensure there are no conflicts when we tie things together, however there will most definitely be gaps in the used addresses (i.e. we may use MPI addresses 1,2,6,7,10 ... etc). Does this present any problems as far as efficiency of communications is concerned - in other words am I going to experience communication errors and timeouts due to polling for nonexistant stations?

 

For MPI, masters should be assigned adjacent addresses in order to increase the efficiency of the network. Slaves can take any address (gaps are allowed), however should not stay between two masters.
HSA (highest station address) should be selected as small as possible.
Of course, transmission rate should be as high as possible.
However, in any case, timeouts are not expected. In the worst case, communication performance can be lower, but not completely stopped.
In Chapter 7 of S7-200 Manual, there is a short, but good explanation of MPI/PPI networks and performance.

For Profibus DP, since there is only one master, addresses of the slaves does not effect the network, provided that that slave exists (master does not look for a new master to join the network).

 

For the best response the addresses should be kept contiguous as far as possible, however the system will only look for addresses it is set up
to look for and no errors will result if the addresses are not contiguous. What do you plan to do with the MPI addresses?

Regards
Donald P

 

Thanks for the info. We are looking at using the DP connection for distributed I/O and using MPI for peer communications between the separate CPUs.

 

You were advised by someone else, correctly so, that you must reduce the highest station address number in the Profibus to reap the maximum
advantage from the contiguous arrangement of the slave ID's.

Regards
Donald P

 

Why would you use MPI for PtP. If both of the CPU's have DP ports you can just shared values on PIW / PQW by using a MS (master Slave) or DX (direct data exchange) for master master. It is much less hassle than PtP and allot quicker to configure.
Any questions just e-mail me ([email protected]).
Regards,
Paul Pierce.

 

HI All

Is it not a bit of a quantum leap to use multiple masters on the Profibus for exchanging data when the MPI is available and also easy to set up for global data exchange? I would rather use Ethernet for the data exchange but if I had to choose between using MPI or having multiple masters on my DP network, I would go for the MPI.

Donald P

 

Hi Donald,
I respect what you have to say on this. I guess I would preferr Profibus over Ethernet. Probably because I do alot of work with Siemens.
But I still think that shared momory addresses PIW's / PQW's are alot easier to write into on a multiple master system than to use xsend and so on over MPI.
Not to mention that you are limited in terms of speed on MPI too (yes, DP is a little slower than ethernet).
Then again this is just my opinion.
Regards,
Paul.

 

Hi Paul and All

I can claim to work only on Siemens S7 these days, that is not only by circumstance, but by choice too.

I am glad we can respect each others opinions, it is so quiet on this subject, it seems as if only you and I have an opinion, and that is sad. I must make a note to try and make some more converts to the one true controller, S7.

Wonderful to have these options if you think about it, not too long ago there would have been little debate on which method is easier, as there
were few options and they were all D-Difficult.

Have you any experience of the IE/Profi gateway device, I have it in my mind to explore the possibilities that this interface offers from the
point of view of diagnostics and maintenance simplicity of the IE in preference to Profi.

Cheers
Donald Pittendrigh

 

Hi Donald,
Unfortunately I think that most people think that people like us who use S7 are kind of odd for sticking with something that can seem so difficult.
I like Siemens, partly because they put alot of time and investiment into the PLC/Automation software and hardware (why shouldn't they!!!).

I think you are referring to some of the new ProfiNet hardware modules. This is all part of the Component Based Automation area.
Seems like Siemens are moving in the right direction, using two mediums that are best suited to what thery do.
I have not had a chance to use any of the new stuff so I can't comment on it, but I have read a bit on it.
People haven't been to keen to take it on, but I guess that will change......slowly.
I would like to hear from anyone who is though. It is always a little easier to convince people to change when you have some testimonials.....

Thanks all and hope to hear from you!!
Paul.