Discussion about DP Coupler Siemens

Hello everybody,

Last few days I have been trying to understand interconnection between two master profibus networks. I have one application in wihich on one side is Step7 project but unfortenatelly on the other is PC based system which I can not monitor. Can somebody please upload one Step7 project in which DP/DP coupler is used for exchange data between two mater profibus networks.Such example would help me a lot to understand everything.

Thank you.

From your post I presume you can look into the step 7 side. If you open the hardware profile you will see the I/O devices of the PLC, inclused will be a DP Coupler.

Example below:

The picture shows the DP's hanging off the profibus, at the bottom the internal set-up for the highlighted DP.

The internal inputs and outputs are configurable as you want, to the PLC these are just I/O. Internally the PLC can just access them as normal input and outputs, alternatively you can have blocks called to multiplex data.

The other end the device (in your case the PC) would need to match opposite exactly the PLC configuration, as an Input to the PLC is an Output from the PC and vice versa.

In the end the DP acts like multiple input/output connections between devices, it needs no software comms to work, much like a normal input or output does not, you just need to set or read the I/O addresses.


Hello and thanks for replying,

I think that I understand now, but just to be shure I am posting HW configuration of DCS that we have:

On the other side we have QCS ( quality control system). Since the firm that delivered DCS wanted to isolate their system from QCS system and they had only one spare place in the rack they decided to put CP342-5 which will communicate with QCS throught DP/DP coupler. They made one FB with for communication and used FC1 and FC2 to send and recieve data (8 IW, 8 QW and 2 statys bytes that are used). Since the DP/DP coupler is only slave on the profibus network I understand communication between CP342-5 and DP/DP coupler. Question is: if there were some more slaves on the bus how could the DP/DP know that the data is sent to it?
On the other side QCS side we have PC with Hischler card (Profibus) - ET200+AI/AO+DI/DO and DP/DP coupler. In this case what was not clear to me was the adressing part. After your reply I think that I understand that the if I put the D
P/DP on the for example Master system 1 it`s inputs and outputs are going to get unique adresses on the bus. That is why I asked for example project with code just to be shure.

One more time thank you for replying


Hello Krcedinac.

You have put in a CP342-5 + DP/DP coupler only to make the connection between S7 and DCS.
Let me suggest to you to cut out the DP/DP coupler and let the CP342-5 be setup as a slave. The result will be the same. S7 and DCS will exchange data via i/o data areas.

Can the DCS communicate via OPC ?
In that case you dont have to let the CP342-5 be a slave, but you can exchange data from master to master.

What about the onboard MPI/DP interface ?
You can also use this if it is not being used for other things.
Especially if you can use OPC, you can set it up as MPI or Profibus and exchange data that way.

Hello Jasper,

Both systems DCS (not real, but called DCS, the PLC which configuration I posted and that controls the paper machine) and QCS (system that controls paper moisure and gramage and does corrections) are in warrnty period. That means that I can not change anything and I am only trying to understand how everything is functioning.Also I do not have tools and knowledge in order to change anything in the QCS system.

If I understand well, if you were constructor of the system you would cut out the DP/DP and use only CP342-5. Does this means that I should have 2 CP`s (one for each system) and then communicate via FC1 and FC2?
Considering OPC... I dont know anything about that variant and if you have time I would appriciate if you can explain me basic things or point where me to look up.



OK, I think I understand better now. DCS and QCS are both S7 PLCs, and you are in charge of the one called DCS.
"DCS" is often used for Distibuted Control System, i.e. a system for controlling large plants, with redundancy and such things. Thats why I jumped to think you were interfacing to a DCS.

For sure, if you are not going to hang anything more on the Profibus line between DCS and QCS then all you need is one CP in each system.
There are many possible variants.
One is master-slave where the data transfer is done via i/o addresses.
Another is master-master where the data is done by messaging (PUT/GET or SEND/RECEIVE).

The only need for a DP/DP coupler is if your PLC is a DP master (maybe with DP slaves), and it shall be unaffected with what happens on the other side of the DP/DP coupler. It could be used for example if you would put the connection on the other Profibus network (your DP master system no. 1).

Anyway, you would have to at least do some minimal changes in the QCS system in order to setup the comms, no matter which solution you choose.


Hello again Jesper,

Let`s continue. Here I am posting part of the code of the FB for data exchange between the CP342-5 and the DP/DP coupler:

So, the 16#100 (256) is the CP342-5 adress and it is sending 34 bytes and also recieving 34 bytes. Since the DP/DP coupler is the only device that we have on that network the data can go to and be recieved from DP/DP coupler. If we had more devices on this profibuse network what would happen. So far I dont see how this data that is being sent is adressed to the reciever. If you can please tell me where. I will post also CP`s propertie


Hello again.

If we had more devices on this profibuse network what would happen.
Then you must setup the additional slaves and assign them i/o addresses. When you download the HW config to the CPU, you also download the configuration of what i/o is assigned to which DP slave.
If you create a contigous i/o area, without gaps (Slave 1 = I20.0..I21.7, slave 2 = I22.0..I23.7), then you would only have to program one pair of DP_SEND and DP_RECV. DP_SEND and DP_RECV actually does not send anything on the Profibus network, DP_SEND and DP_RECV sends the data to the CP, and the CP in turn sends it to the configured DP slaves.
If there are gaps in the i/o area for the DP slaves (Slave 1 = I20.0..I21.7, slave 2 = I30.0..I31.7), then you would have to program several DP_SEND and DP_RECV.

I take that everything works OK by now.
In stead of posting so many screenshots, you can archive (zip) the project and post it here.

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