Advantages of profibus over MPI ?

Hi, i want to know the disadvantages of communicating through through MPI port of SIEMENS PLC to RS-232 port of PC over, using PROFIBUS.I want to know details regarding the performance of these two.I will be waiting for your kindly responses.thank you.

Hello ragav_rp;

Let's presume you are speaking about programming and/or monitoring data from the PLC.

Here are a few points to consider:
- speed: MPI works between 19,2/38.4 kB, with a PC adapter on COMx port of the PC, to 187,5 kB on a dedicated CP card in the PC (e.g. CP5611, Cp5511, CP5613). The MPi port can be setup at 12 MB (as a DP port, though). Profibus can be used at 12 MB (with the same PC cards) although generally they work at 1,5 MB.

-distance; MPI is limited to 50 m (segment length); you can use repeaters to extend it, as shown in the following interresting illustration:



Profibus allows a segment length of 100m at 12 MB, 500m at 187,5 kB; segments may be added with repeaters.

Although not interresting for programming purposes, this information is also a good distnction between the 2 buses:
-number of participants: for MPI, up to 31 stations total on all segments. For Profibus, you can have 31 stations per segnment, with a total of 126 stations on the bus.


Hope this helps,
Daniel Chartier

I'm not sure if this is correct but I've always been told that there are issues in connecting two or more PLC's via Profibus and that the comms can be slightly unstable. However connecting two or more PLC's via MPI is fine.

Again, this is just what I was told by a rep at Siemens who said that if you just wanted to run 'slaves' from a PLC such as Inverters etc then Profi is fine but if you wanted multiple PLCs connecting then MPI was better.

So a typical config would be PLC's from many machines connected via MPI and then each individual PLC would have its own nodes (such as inverters etc) connected via Profibus.

Someone with more knowledge on this though will probably be able to confirm whether I'm just speaking out of my hat...

;-)

Hello, Johnny T.

I've done a couple of multimaster installations over Profibus-DP, and never had stability problems, although you do have to evaluate the amount of data you need to transport over the bus. What has given me problems was when I overlooked the connection limits of the older S7-300 CPUs and tried to put too many HMI stations on a single bus along with the slaves. This is easier to overcome now, as the newer CPUs support more connections over DP.
Hope this helps,
Daniel Chartier

Daniel

Thanks for that. I was never 100% sure on the differences myself between MPI and Profi.

Personally I find MPI easier to set up.

Thanks for the info.

Cheers

JT