S7-400H MODBUS TCP possible connection numbers

I would like control 11 independent facilities via MODBUS TCP. Our requirement is making Fault-tolerant system. So, we will use two 417-5H CPU modules. And we are considering using CP443 cards.

So, I am considering making rack as attached drawing.

In this kind of requirement, is it possible to run 11 different MODBUS TCP communication?

Hello Dolpong;

To complete your project you will need a redundant license for Open Modbus/TCP.

The attached illustration was taken from a Siemens brochure on Open Modbus/TCP.

This FAQ will give you mere information:

https://support.industry.siemens.com/cs/ww/en/view/22660304

Hope this helps,

Daniel Chartier

Dear all,

I had asked possible connection numbers with MODBUS TCP communication. And I have a problem now.

I made configuration as my previous diagram. For your understanding, I am attaching the same diagram again.

And we ordered Modbus/TCP RED V2 (2XV9450-1MB11) 

We would like to use PLC as MODBUS client. And would like make connection with 11 different MODBUS servers.as attached diagram picture

After reading "SIMATIC Modvus/TCP Redundant Communication via CP443-1 in H-Systems" manual, I came to think that "MODBUS/TCP RED V2" cannot make different 11 MODBUS connections simultaneously. Then, we cannot make configuration as attached diagram.

If we move PLC MODBUS TCP connection to CPU's PN port and use Modbus/TCP PN CPU Redundant (6AV6676-6MB10-0AX0), could we make 11 independent MODBUS TCP connections ?

Hello Dolpong;

What information are you basing this estimation on? There is a huge amount of documentation on the Siemens Support site that can answer your question quite precisely.

How many devices can you connect to an S7-300/S7-400 using the Modbus/TCP protocol?

https://support.industry.siemens.com/cs/ww/en/view/103709646

Communication via CP443-1
One connection is occupied for each connected Modbus/TCP device.
The maximum number of connections - and therefore the maximum number of Modbus/TCP devices that can be connected - is given in the technical data of the CP under "Performance data / Open communication". The number of possible connections for open communication or via SEND/RECEIVE blocks specifies how the maximum number of Modbus/TCP devices that can be connected to the CP443

The CP 443-1 (6GK7 443-1EX30-0XE0) technical data shows 64 connections possible  in the section Performance Data/Open communications (with T-blocks and the Send/Receive blocks). So your 11 servers should have no problem connecting to your S7-400H system over Redundant Modbus/TCP.

https://mall.industry.siemens.com/tedservices/DatasheetService/DatasheetService?control=%3C%3Fxml+version%3D%221.0%22+encoding%3D%22UTF-8%22%3F%3E%3Cpdf_generator_control%3E%3Cmode%3EPDF%3C%2Fmode%3E%3Cpdmsystem%3EPMD%3C%2Fpdmsystem%3E%3Ctemplate_selection+mlfb%3D%226GK7443-1EX30-0XE0%22+system%3D%22PRODIS%22%2F%3E%3Clanguage%3Een%3C%2Flanguage%3E%3Ccaller%3EMall%3C%2Fcaller%3E%3C%2Fpdf_generator_control%3E

If this is not the exact model of CP 443-1 you plan to use, look for the datasheet for that model so you may have the correct information.

Hope this helps,

Daniel Chartier

Dear Daniel, Thank you for your comments.

In order to implement MODBUS Client at S7-400H system, we ordered  Modbus/TCP RED V2 (2XV9450-1MB11) 

Please refer attached manual file that I found out at Siemens website. And I think that FB MB_REDCL could be used.

In order to make client program to distinguish one connection among 11 connected  servers, I think that remote server device identifier(such as IP address) is required to select communication partner. But, FB MB_REDCL does not have such parameter.

Could you check my problem  and let me know whether I can find out solution?

Thank you for kind advice from experts.

Please check further explanation on my problem.

If you could give us solution for my problem, it would be very helpful to me.

Best regards,

  Jay Park

danw

I'm a former SCADA guy and instrument guy so I've done a lot of Modbus, but I can't answer Siemens specific license or functionality questions because I deal with Siemens field instruments, not Siemens controllers and comm modules.

1)  > 11 MODBUS connections simultaneously.

In general, a Modbus client never has 'simultaneous' connections.  A Modbus client always processes however many transactions in a sequential fashion, one after the other:

In your multiple server (slave) topology, the Modbus client (master) could   
- execute a read transaction or write transaction to a given server (slave), wait for reply or timeout, then
- execute a read transaction or write transaction to the next server (slave), wait for reply or timeout, then
- execute a read transaction or write transaction to the next server (slave), wait for reply or timeout,
and so on until repeating the sequence.

The Ethernet/485 converters each have a unique IP address so each 'transaction' addresses a separate converter using its unique IP address.

2)  >"MODBUS/TCP RED V2" cannot make different 11 MODBUS connections simultaneously

Why do you think this?  What statement lead you to that belief?

I ask because I have the impression that although Siemens is late to the Modbus game, they are a player and I would think that a Siemens Modbus client/master processor could handle 11 server/slaves.  That's not a particularly challenging Modbus application.

Is the question the quantity, 11, or is it the nature of simultaneously connections, whatever that means to you?

3)  There are generic serial servers (Ethernet-to-485) converters and serial servers specifically for Modbus.  I would recommend a converter that was designed or Modbus, because that's what you're doing.

 Thank you dnaw,

1) Regarding sequential logical process among physically multiple connection, you are right. I mean "simultaneous physical and TCP/IP network connection".

2) When I read the manual, the FB MB_REDCL does not have remote server identifier field(such as IP address) And MB_REDCL use connection ID that is fixed at NetPRO.

3) We will use MODBUS TCP/RS-485 converter. 

If you have suggestion to solve my problem, please let me know.

Thank you.