Profibus, DP, and MPI

I have a customer that wants us to use a S7-400 PLC. I don't get into Siemens too often (unless I have to). This machine doesn't do much and needs minimum I/O (turning on/off a motor, passing 1-2 analog signals)

The only S7-400 PLC with profibus is $15k! The lowest level module is only about $2k, but has DP/MPI.

From what I can tell, MPI is a subset of profibus that is more limited, but the connector is identical. Can the lower-level MPI/DP connect to a Profibus network without issues or will I need the (much) more expensive CPU?

Thanks in advance!

EDIT: After some more research, I think my confusion lies in the fact that the Siemens part number listed MPI/DP, but DP is just short-hand for Profibus-DP. For some reason in the TIA selection tool, it only says "Profibus" for the 416 CPU and DP for the other modules which made me think it was a separate network protocol. When I look at the datasheets for the 416 and 412, both mention MPI / Profibus-DP. If I am wrong, please correct me!

normally the S7-400's have 2 interface connections- X1/X2

X1- This is normally a MPI/DP Port (MPI and Profibus DP), by default it is set to MPI but can be set to Profibus DP in the hardware configuration.

X2- This is normally a Profibus DP port only.


Regards

Daniel

Be aware that some older versions of S7-400 hardware are not compatible with TIA, I had this in 2012, we had a program running on an old 400 (circa 2003) when I loaded the project into TIA I could monitor but not make changes as the hardware version was too old. The reason I'm posting this is that we were running S7 V5.3, our computers were replaced & Win7 installed, S7 V5.3 would not work, our Siemens supplier told us to upgrade to TIA V11, this is when we found out we could not make changes on our 400s We then had to buy S7 V5.5 as the TIA was opened we could not return it even though it was on a recommendation on their part.

Rson said:

I have a customer that wants us to use a S7-400 PLC. I don't get into Siemens too often (unless I have to). This machine doesn't do much and needs minimum I/O (turning on/off a motor, passing 1-2 analog signals)

Click to expand...

Have they given a reason for using the S7-400? Seems like you could save a lot of trouble by using an S7-1200.

Rson said:

EDIT: After some more research, I think my confusion lies in the fact that the Siemens part number listed MPI/DP, but DP is just short-hand for Profibus-DP. For some reason in the TIA selection tool, it only says "Profibus" for the 416 CPU and DP for the other modules which made me think it was a separate network protocol. When I look at the datasheets for the 416 and 412, both mention MPI / Profibus-DP. If I am wrong, please correct me!

Click to expand...

DP means Profibus, yup! MPI uses the same RS-485 physical medium but is a simpler protocol that was commonly used back in the day for programming ports. Most Profibus PLCs support MPI as a mode on at least one of the Profibus ports, but it's just an option.



There is also another flavor of Profibus out there called Profibus PA, but that never plugs directly into the PLC, only via a gateway. It's for things like intrinsically safe comms to devices.

If you use Profibus/DP, you need MPI adapter 6ES7972-0CB20-0XA0 to program S7-400 PLC.
MPI adapter on EBay costs about $30, working just fine.

mk42 said:

DP means Profibus, yup! MPI uses the same RS-485 physical medium but is a simpler protocol that was commonly used back in the day for programming ports.

Click to expand...

That is why it isn't a simpler protocol. MPI contains means to program a PLC whereas Profibus does not.
ProfiNet is the way to go. Some of you might remember how I would curse Siemens for their difficult to use, slow and limited I/O. That all changed with ProfiNet.

Peter Nachtwey said:

That is why it isn't a simpler protocol. MPI contains means to program a PLC whereas Profibus does not.

Click to expand...

You can program the plc over Profibus.

L D[AR2 said:

You can program the plc over Profibus.

Click to expand...

There is nothing to keep Siemens programming software from adding the same headers that MPI uses.
Normal Profibus packets are pretty simple.

L D[AR2 said:

You can program the plc over Profibus.

Click to expand...

The physical layer is the same for Profibus and MPI. For PLC programming it would still be MPI protocol, It is possible to mix PROFIBUS IO and MPI nodes, but it is necessary to adjust all communication parameters to MPI limitations, I think.

Show us a document where mpi and profibus can be mixed
And a reason to do so.

I do not have time to look for such document, but I can share my experience.
First, reason to do this? Let's say you have a network in which there are more than one Siemens PLC, and on top of the Profibus DP communication the PLCs need to exchange PLC peer data. or maybe there is an HMI somewhere deep in some room in which there is no Ethernet infrastructure. In that case it may make sense to use the Profibus cable for both Profibus DP and MPI. Of course doing this requires deep knowledge of what needs to be done in terms of bus timings and so on. Things nobody needs to care about when all you have is one Profibus DP master and a number of Profibus DP devices.
I am sharing below a screenshot of a network I was called to troubleshoot many years ago at a factory that uses machines designed in Europe. Unless the Softing diagnostic tool is wrong, I can see a mix of Profibus DP devices and MPI devices.

And I’ll slow you a ProfiTrace scopeware tab of all the errors and dropouts
Non redacted

I'm sorry, I do not understand above post #13. I thought you wanted to know why I said that MPI and Profibus DP can work simultaneously.
By the way, I think Profitrace is a great tool

Im saying trying to run mpi and profibus protocols on the same wire is a terrible idea
Ive never heard or saw it done