Location to install RS485 Repeater in the Profibus DP network

Hi Everybody!

My system includes PLC S7-400 (CPU 414-3DP is a DP master) and DP slaves ET200S. The distance between master and slave is 280m. I use copper cable to connect DP master and DP slave. To ensure stable operation of the system, I should install the RS485 Repeater (6ES7 972-0AA02-0XA0) near master or slaves?

Hope to get everyone’s help.

Thanks All!

Hello MrBin;

Your question is interesting, and shows how a misunderstanding of the use of a repeater can bring about network problems.

First: the placement of a repeater has nothing to do with the location of the Profibus network master.

Next: The questions you must ask are: what is the total length of your present network (without a repeater, the network constitutes only one segment); how many nodes (master and slaves) coexist on that Profibus segment; and finally, what speed do you want your Profibus network to work at.

You only need a repeater if your answers show that one (or more) of the conditions specified in the following document is not met.

What should you watch out for when using the RS485 repeater in PROFIBUS and MPI bus systems?

Say your network is 280 m in total length, from the first to the last node, there are 10 nodes in all and you want to run at 1,5 MB. The link above shows that the max. length of a segment at that speed is 200 m, so you will need a repeater to extend the network into 2 segments; the other parameters of length and number if nodes are within the values for a single segment..

You can place it anywhere it is convenient, as long as it allows no more than 200 m for any segment. It could be exactly at the middle point (2 segments if 140 m), or with one segment of 80 m and a second one of 200 m, or one segment of 180 m and a second one of 100 m.... 

Note: use only the data that concerns a Profibus network in the document linked above.  MPI has different requirements.

Hope this helps,

Daniel Chartier 

Hello dchartier,

Thanks for your helpful information!

My Profibus-DP network have transmission rate: 500Kbps; and network diagram as attached picture.

Should I use a RS485 Repeater?

Best Regard.

Hello MrBin;

Consider the copper Profibus network we have information on:

- there are less than 32 nodes on the segment;

- length of Profibus cable is 380 m;

- bus speed is set at 500 kB, which allows for a segment length of 400 m.

So for that segment as described you should be able to operate without a repeater.

My question now is: how many independant Profibus networks are configured on this PLC? Is there a separate DP master for the second segment shown on your diagram (without details), or is it also part of the same DP network?

If it is part of the same network, your numbers (on length and number of nodes) must be re-evaluated to determine if you need a repeater.

Hope this helps,

Daniel Chartier 

Hello dchartier,

Thanks for your help, I have more knowledge about Profibus network.

I will configure my Profibus DP network with transmission rate: 500Kbps, and detailed network diagram as attached file.

Best Regard.

Hello MrBin;

I have a few questions on your network design:

-You have installed a DP/DP coupler in node 23. Only one slave is configured. Is there an external DP master connected to the second slave on the coupler? If not it really serves no purpose...

- in your network you are using 3 repeaters, which seems to be overkill for this application. It seems that you could configure your network with only one, as far as number of slaves/segment  and bus speed requirements.A total length of 400 m and 29 slave nodes could be handles without any repeater, at 500 kB. One repeater would be necessary if you want to use a bus speed of 1,5 MB.

Two of these repeaters are connected directly to one another, back to back. Any reason? You could easily eliminate one (and possibly the last one at the bottom) by reorganizing the cabling of the network, maybe by thinking of the attached passthrough setup for the repeater segments.

This screenshot is taken from the RS-485 repeater manual:

https://support.industry.siemens.com/cs/us/en/view/48598071

Hope this helps,

Daniel Chartier

Hello dchartier,

Yes, there is an external DP master connected to the second slave on the coupler.

My cabling of the network is already installed, I can’t reorganize; so I have to use 3 repeaters.

I configured Profibus DP network for my application according to the Profibus_network_diagram (above). Until now, my system stably run.

Thank you very much about your help!!!

Best Regard.