S7 Profibus Comms


Thread Starter


I am attempting to send data between some S7-315's, upto say 50 'words' of data per PLC (bits, words, ints, and real values)....

Between two of the PLC's, I have configured 5 links of type FDL. The problem I am having, is that I dont seem to be able to transfer (successfully) more than two bytes of data per AG_Send (FC5) command.

The reason why I configured more than one link, was because I need to send data from 5 different datablocks (some of which are of different datatypes), and could not understand how the PLC's would know which AG_Send block belonged with which AG_Receive block. I dont know if I am assuming coreectly, but my assumption is that for each link, the two connected PLC's can have one send command, and one receive command - am I right???

Can anyone tell me why I only seem to be able to succesfully send 2 bytes of data???

Also, am I using the right connection type (FDL), somebody said to configure the PLC's as Master, and Slaves, but I need to transfer data between all PLC's, and was affraid that if the Master failed, then there goes all the communications???

Maybe I have been looking in the wrong places, but the Siemens Help files, and examples that I have looked at don't seem to be easily modified into a real-life application - or maybe I am just looking at it from the wrong angle???

Can anyone point me in the right direction - please...

Jeff Winborn

Sorry to read that you're having problems. To set up data exchange between plc's you'll have to edit the file called 'Connections' in Simatic Manager. For communications between S7 plc's, I use the 'S7 Connection' instead of 'FDL' but that might not make a difference. When you set up these links between plc's, there will be a local ID and a partner ID that are unique. These ID numbers are needed when configuring the AG_SEND/RECV blocks. This is how plc #1 knows it's sending/receiving data from plc #2. You'll need to download the 'Connections' file to the associated plc's. Also, the data length should match exactly what you're trying to send/receive or problems can occur. Hope this helps.

Jeff Winborn
[email protected]

If there is nothing else on the profibus network the simplest is to config the network as MPI.

1. Right click on define global data in manager
2. Add each of your CPUs by right clicking on large grey box.
3. Add the data to transfer
4. Mark the required master for each data transfer. One master sends to many slaves

This only works in MPI !!! hurry up siemens ETHERNET!!!

Donald Pittendrigh

Hello All

I hear the plea for Ethernet, but I am not sure why.

Firstly, Profibus is very fast for transferring small blocks of data between PLC's, and to boot it is deterministic, requires little or no extra programming or hardware, oh and did I say, it's fast.

In order to transfer a few bytes of data Ethernet requires a very complicated structure of acknowledgement and header transfer that is not
required with Profibus, and is consequently only efficient on large blocks of data. With Profi the data is possible to move by hardware settings on the interface modules alone and so no software in the CPU required, that's when it is fastest.

If you have a Profibus interface, to use global data on MPI is nice and easy, but it is second best, big time!!!, make the one CPU a Profibus
slave and the other PLC a Profibus master, then connect the two Profibus interfaces to the same Profibus network, even if you have other I/O
stations on it, now go to either NCM (connection manager) or open the hardware configuration and double click each Profibus interface and set up the addresses you wish to use and define them as in or out's.


The data is available by addressing the Input and Output areas by normal means.

As far as the "come on Ethernet" goes, which type of Ethernet is it that you would like from siemens, all the kinds flavors and variations I know of, are already available, I have personal experience of S7 functions, which is proprietary, but also UDP, TCP/IP and ISO/OSI, all of them work
beautifully with Siemens, there are also FTP, mail server connections (probably SMTP) but I have never used it so am not sure, HTTP from the
IT version of the H1 interface module, there is even a full suite of products to talk to Unix and Linux, so what is it that you are waiting for?

Donald Pittendrigh