SCADA system specification

Process control authority Bela Liptak brings in specialists from his cadre of co-authors to answer a reader's question. Find out where to obtain a general specification of a SCADA system.

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QUESTION:

 

Paul Bulmer, Modern Process Control & Custom Panel Shop


ANSWER:

 

  • General SCADA Requirements categories might include:
  • System availability and failure recovery including redundancy
  • Maintainability including a proven support track record for the vendor/system
  • Compatibility with existing systems to capitalize on existing infrastructure
  • System performance including scan rates, response times, and system loading
  • Security, for obvious reasons
  • System expandability including the ability to extend all requirements of the system and the use of open communication protocols
  • Data retention and management, in support of regulatory requirements such as 21CFR11 Compliance

A specific SCADA sub-section in a URS for a packaged vendor supplied process automation skid that is to be connected to a site SCADA system might read as follows:

1.1.1 Supplier shall exchange data with the site SCADA system in the form of a data map that presents the following data from the site SCADA system:

  • All Inputs and Outputs (I/O)
  • Alarms
  • Interlocks and Permissives
  • Recipe Parameters (Pressure Set points, motor speeds etc.), if applicable
  • Modes
  • Batch Execution Events (Start/Stop/Hold/Restart etc.) if applicable
  • Key Process Parameters (such as Min/Max pressures, sample times etc.)
  • Batch End Report Parameters if applicable
  • Other Events (i.e. operator actions, system events)
  • Communications Watchdogs

I suggest that you investigate whether your local ISA chapter sponsors local classes on SCADA as I am aware of local chapters that do provide formal training courses on the subject.

Additional places to look for information:

  • GAMP JETT Forum
  • IEEE including standard C37.1-1994, "IEEE Standard Definition, Specification, and Analysis of Systems Used for Supervisory Control, Data Acquisition, and Automatic Control"
  • Online user discussion archives such as “The SCADA Gospel” 
  • White papers, case studies, and specifications can be found from many of the major automation suppliers such as Citect or Emerson Process Management

Scott Clark, Senior Engineer, Automation & Information Services, Merck & Co. Inc.

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  Need an Expert Opinion?
Send your knotty control and automation questions to Béla Lipták at bliptak@putman.net. Lipták will either answer the questions himself or bring in specialists from his cadre of Instrument Engineer’s Handbook, Fourth Edition co-authors. Questions and answers will be published online on www.ControlGlobal.com. Each month, Lipták will pick the best question, and publish it, along with the answer in CONTROL. If your question is chosen, a prize will be awarded.
I'VE INCLUDED some references at the end of this discussion that I have found useful but please let me try to expose some of the differences between SCADA and DCS systems even though they do share many similarities in components and requirements. I’ll also try to touch on some general and specific requirements of a SCADA system. Redundancy in a SCADA system is typically handled in a distributed manner whereas DCS redundancy is handled in a parallel component manner. SCADA systems are typically event based and tend to be more complex when it comes to holding the last good values from an RTU (Remote Terminal Unit) and reconciling the data coming from traditionally less reliable transmission media. In light of these differences, DCS specifications may be consulted to help ensure that your SCADA specification is complete.
I AM a member of ISA and I am trying to locate a general specification of a SCADA system. Do you have one I could use or do you know where I could obtain one?
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