Emerson Exchange 365

MODBUS - Multi Masters

Hi Experts,

For MODBUS communication, general MB slave can respond one master only. When there are more than one master acquiring data, how does DV deal with this situation?

For example, there is one APC PLC and two DV controllers. When both DV controllers are acquiring the pressure reading in APC unit which is tied into APC PLC, how to configure this kind of requirement? One way I can do, however, appoint DV controller A as the master to pull the reading PT1 from APC PLC via MODBUS and controller B refers to PT1. But the question is, what about controller A is down, controller B will lose PT1? 

In a client-server modeled DCS system (can I say DV system is a peer-to-peer model?), there is a type of points called "server point" which is built and controlled by DCS server, all third party data are in a data pool, all DCS controllers could visit the pool for the same data acquisition at the same time.

Thanks.

Jiye Yuan

• 23 Feb 2018 3:34 PM
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13 Replies

• salehatiyyat

  

  • 23 Feb 2018 3:44 PM

  Hi Jiye,
  May I ask why does it have to be like this? Why not to let the plc only act as the Master. And then integrte the PLC with your DCD system.

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• Alexandre Peixoto

  

  • 23 Feb 2018 4:15 PM

  The Modbus-TCP protocol (over Ethernet) supports multiple masters in the same network.

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• Jiye Yuan

  

  • 23 Feb 2018 4:16 PM

  In reply to salehatiyyat:

  Because the DCS's which I have used can only be the master which acquire the data rather than respond PLC which in most case is working independently.

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• Steve Linehan

  

  • 23 Feb 2018 4:20 PM

  There are various options available to you to achieve this. The configuration you select will depend on assessing failure of components and what will happen in each case. As you stated if you let controller A get the data and pass to controller B then you have a single point of failure that will cause data loss to both. Various options exist.
  1. DeltaV supports redundant Controllers and redundant serial cards. So no controller loss needs to be considered.
  2. As suggested by Salehatiyyat the PLC can be master and send data to both Controllers.
  3. Most PLC systems support multiple comms cards as an option so both Controllers can get the data independently.
  4. There are some commercially available modbus arbitration devices that can allow multiple masters to read data. You can Google multiple modbus masters.
  5. Protocol converters can also be utilised to achieve the same type of data transfer.
  6. You could also use modbus over TCPIP (If this is an option with the PLC) to allow multiple masters.
  
  In short there are various options available but most require additional hardware to achieve what you are trying to do. To answer the question regarding “server point” this is not really a DeltaV question as Modbus RTU is a simple master slave arrangement. If a configuration for a server point was to be configured it would be at the PLC end not within DeltaV.

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• Jiye Yuan

  

  • 23 Feb 2018 4:28 PM

  In reply to Alexandre Peixoto:

  Thanks Alex!
  That's what I am trying to figure out in DV system. Based on what I know about DV system so far (I am just a beginner for DV system), to talk to an MODBUS node, a communication module has to be installed under a controller. How to configure in DV controller to apply MODBUS TCP/IP? What kind of hardware and software do we need? Does it require a special license? Thanks.

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• Jiye Yuan

  

  • 23 Feb 2018 4:37 PM

  In reply to Steve Linehan:

  Thanks Steve.
  Just let you said, require additional hardware(s) to achieve, that's all I want to know. Just wondering if there is a solution based on software only. As DV system doesn't have "Real-time Data Server", a software-only solution sounds like impossible.

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• salehatiyyat

  

  • 23 Feb 2018 4:38 PM

  In reply to Jiye Yuan:

  is there any control physiology or it is just monitoring?

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• Jiye Yuan

  

  • 23 Feb 2018 4:49 PM

  In reply to Jiye Yuan:

  The reason I come up with this discussion is when we do the existing system expansion, we may add a new DV controller which may need to talk to the same PLC which is already embedded in the existing DV system. If using existing DV system as a pass, there will be a data lost risk when the parent controller is down for maintenance. Just trying to figure out a check list to deal with this kind of situation.

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• Jiye Yuan

  

  • 23 Feb 2018 4:55 PM

  In reply to salehatiyyat:

  A simple example is the factory utilities unit or ASP unit in a refinery plant. The data from these units are sharing through out the entire plant.

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• salehatiyyat

  

  • 23 Feb 2018 5:36 PM

  In reply to Jiye Yuan:

  Thanks. Try to consider intergating your PLC to thr DCD you can get access to all data in the PLC and you can avoid the two masters issues.

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• Alexandre Peixoto

  

  • 23 Feb 2018 6:03 PM

  In reply to Jiye Yuan:

  Hi Jiye,
  Modbus-TCP is available with DeltaV Ethernet I/O Cards (EIOC) on v13.3 and higher, or DeltaV VIM cards. The carrier-based serial card you are referring to would only provide Modbus-RTU serial comms (which as Steve mentions, is a simple master/slave protocol). You could still try to get Modbus-RTU to TCP/IP converters and use the serial cards, but I'd say the EIOC and VIM cards are much better options to accomplish your goals - below are the product data sheets for your information:
  
  www2.emersonprocess.com/.../DV_PDS_DeltaV_EIOC.pdf
  
  www2.emersonprocess.com/.../PDS_M-series_VIM2.pdf
  
  www2.emersonprocess.com/.../DV_COL_PDS_S-series_VIM2.pdf
  
  I hope this helps!
  
  Regards,
  
  Alexandre

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• Jiye Yuan

  

  • 23 Feb 2018 6:49 PM

  In reply to Alexandre Peixoto:

  Thank you very much! Alex

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• Jiye Yuan

  

  • 23 Feb 2018 6:51 PM

  Thanks everyone for discussing. Have a great weekend!

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