Provided the connection IDs are unique, you can enable multiple connections to the same MB  node address (IP for MB/TCP).

The tricky part is to make sure that the 2 MB/TCP sends are not able to interrupt each other mid transmission.  This is rarely an issue, but if you are using an OB3x to send MB/TCP requests at a precise rate, you need to make sure to disable interrupts before each MB block calls - and of course, enable interrupts right after each MB block call.

Robijn

Hi everyone!

I'm currently working on a project where a customer receives a DB over Modbus TCP/IP.

Everything is tested and works well, but now they requested to make an extra connection to the Modbus server with the same IP and Port number.

They have everything redundant, so they have two BMS systems they need to connect to our system.

The company's IT expert indicated that most devices allow two identical connections over Modbus, but thought this might not be possible with an ET200SP PLC. When they try to connect the other BMS system, the connection is ended.

Does anyone have more information about whether this is even possible? And if so, do I need to make some changes to the server block to achieve this?

Thank you in advance! :) 

Kind regards,

Robin

Hello,

I think that best procedure can be send Support request and return back here with informations from Support. This can be clear then and we all will benefit to what exactly possibilities we can have in case of S7 HW. To my actual experience this isn't possible, only with set of different Port value as independed connection channel. maybe also some IT workaround with managed switch with filter and forward maybe, simply same IP and Port request is ok but after switch it is set to that different Port, I don't know, this can set only  some experienced IT Team, ask them maybe about this.  Also your sitaution is with redundancy what is absolutelly out of my knowledge, only very basic. Support request is then probbaly the best option.

Hi Robin,

Robijn

I contacted Siemens and after emailing back and forth for a while, we came to the conclusion that this is not possible. The only option is to create an identical server with a different ID, but then you have to continuously switch between the two server connections.

They indicated that this is possible with the different inputs and outputs, REQ, DONE, BUSY ect. but these only exist with MB_client blocks and not with the server blocks. They can't provide any further explanation on this, so I'll just leave it at that. 

If anyone has any tips, they are very welcome!

ok, they only confirmed my idea about actual functionality so ok for now. On other side I think that system is logical. I think that many users missunderstand TCP stack (generally depended to OS) or multi instance, parallel, multi threading or specific Server features to handle theoretically unlimited number of Clients that some servers have (any Chat or Facebook f.ex.) but this is really different story in my opinion. No one knows what is behind the code of Server. Really I don't know what to recommend more than I've already in my previous post. My idea about that special IT solution can be maybe ok but you need create another Server block with different Port value and test this. Simply another Client will connect with same IP and Port as is now but behind switch it is changed to your expected different Port on second Server block and traffic will be independed, although it will share same data. This is only my quick idea. If I can say my opinion, your Customer request is simply like big missunderstanding and their IT specialist (you mentioned him) need give them clear informations how this system is working No problem with next talks here and good luck.