Need to know:

What are the limitations of MPI and Profibus DP?

Do I need repeaters for distances over 50 m. when using MPI ?

Do I need repeaters for distances over 50 m. when using Profibus DP ?

There are simple connectors that connects bus cable directly to the port on the PLC, PC or OP. And there are "bus terminals" that connects via a drop-cable.
What are the reasons to choose one over the other ?

Any help would be greatly appreciated.

Jesper Pedersen

 

well you do need a repeater for MPI network exceeding 50m, where as for profibus it depends on the baud rate it can be 1000m! at 19.2k baud.
MPI has a fixed baud rate of 187.5K.
As far as connectors are concerned. You'll need the bus connectors for profibus network for two reasons, one is for multidrop configuration and the other two have resistance terminations ON at the two extreme ends of the profibus. bus connectors are also required for a MPI network of more than two nodes.

 

Hi Jesper:

Little guidelines about MPI/Profibus:

-Physical structure of wiring is the same in both cases(RS485)
-Repeaters needed when the limits of devices per segment, or distance, are reached.
-32 devices per segment (up to 126 per network)
-drop lines add capacitances (must be short and used under 187.5Kb/s).Recommended NOT to use drop lines.

*****Profibus:
*Speed up to 12Mbaud
*Open Standard.Any manufacturer can connect to Profibus
*Plant solution from bit to Cell level
*Various protocols depending on the application: DP,FMS,PA (Ex zones),S7,FDL
*Segment lenght depends on the transmission speed (up to 1000m at 187.5Kbaud)
*cable,fiber optic, infra-red and RF transmission.

*****MPI:
*Up to 12Mbaud with latest hardware
*Siemens protocol, not an international standard as Profibus (less devices to connect)
* 50m max. lenght (use of repeaters if greater distances needed)

Hope this to be useful.

Best regards

Aquilino Rodriguez
Festo Spain

 

> Need to know:
> What are the limitations of MPI and Profibus DP?

* MPI is a subset of profibus and allows only up to 31 slaves with a fixed communication speed of 187,5 baut. Everything possible with MPI can also be done with profibus, only faster. Profibus DP (DP from Decentral Peripherics) is a protocol for profibus with remote IO (Cyclical Master-Slave with 1 master and up to 126 slaves), other profibus protocols like FMS are often used for data communication and HMI (Client-Server, more than 1 server possible). All protocols (MPI/DP/FMS) use the same hardware (RS485).

> Do I need repeaters for distances over 50 m. when using MPI ?
* The use of repeaters depends on the distance, transmission speed and on the number of slaves. Up to 1.5 Mbaut and up to 31 slaves, you do not need repeaters for distances up to 200m.

> Do I need repeaters for distances over 50 m. when using Profibus DP ?
* Only if you use more than 31 slaves, distances up to 100m are allowed up to 12 Mbaut

> There are simple connectors that connects bus cable directly to the port on the PLC, PC or OP. And there are "bus terminals" that connects via a drop-cable.

> What are the reasons to choose one over the other ?
* Shielding, allowable baut rate, mechanical stability, termination.
Termination is a big issue, you can use the termination resistor on the connector but this means that removing this connector or switching off the power to this slave will interrupt your bus!!! Better is to use the active termination and provide it with a redundant 24V power supply.

 

Jesper,
The MPI & DP interfaces use the same protocol - RS485 I believe.

DP is capable of having more connections (127 Nodes I think instead of 31) - And is aimed at the Process Control Sector (Drive/Valve Control) and Distributed Peripheral applications (Remote PLC racks etc)

MPI is for smaller applications and is aimed more at HMI Panel and Programmer connections to a network, I am currently trying to see how pratical it is to use MPI to exchange data with a PC station.

The Bus Speed on both types are configurable - this affects the permissible distance between nodes. The faster the Bus speed, the closer the nodes have to be to each other. I have used a Profibus DP network at 1.5 Mbps in an Industrial environment with node gaps of just over 50m successfully.

I have used connectors that take the bus cable into and out of the plug without any problems.

These plugs are fitted with an 'end of line resistor' that can be switched on if the plug is the last one at either end of the bus cable. You must switch this on at either end of the bus cable to avoid unwanted signal reflections.

I Hope this helps you

regards
Jon Buck

 

Would appreciate some ideas on the following:

What kind of cables are normally used for Profibus? How will different twisted pair cables perform - UTP, STP? Is only lowest per meter capacitance important? what connectors are normally used for stubs( junction points for stations) along the total run of the cables? Is there any comprehensive guide/tutorial to all this other related topics on web?

Thanks for any suggestions.
Pravin

 

Hi Fatnani, Profibus is an open project, but is really 'drived' by Siemens, and as is logic Siemens sell Cable and connector for making a network. I have maked some experiment in my factory for substituing cables and connectors. My results are that until 3 Mbits normal shielded cable and normal sub-d connectors work well, but when speed raise up until 12Mbit/s (the maximum avaiable on profibus-dp) is required a low capacitance cable and special sub-d connectors with added some inductance for squaring data signals. In m factory we buy Profibus cable from Intercond, I'm located in Italy, and connectors from Siemens.

I think that in the world there are dozens of manufactureres of cable and connectors, I've not tried to test profibus with UTP, FTP and STP cables, I think that differencies will get out when you go at 4,5 or more Mb/s. If you have to make this experiment let me results.Thanks.

Best Regards.
Gabriele Corrieri

 

The cable used for Profibus DP is completely specified in the Profibus specification, which can be obtained from the Profibus PNO (www.profibus.com) - this site probably contains description of the cable and vendor contacts. It is essentially high spec twisted pair, and you *must* use the correct type with correct termination and bias resistors or you *will* run into problems. It's false economy to fiddle about with cheaper cable brands.

Cheers

Tim Linnell