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Dave's Top Ten Reasons Why SCADA Systems don't Work

2010-03-28 14:00:31views: 833Dave (

 Dave's Top Ten Reasons Why SCADA Systems don't Work

The purpose of this article is to identify the predominant reasons why some SCADA systems don't work well or suffer from intermittent performance issues. Ever have a problem with your system that you just can't put your finger on? Or you have been living with the problem or know about the problem but do not know what the solution may entail? Or maybe, the customer is just not making full utilization of the system you installed last year. If so, take a look below. You may find an answer or at least an idea where to investigate.

Reason #1. Poor SCADA Communications Performance due to Lack of Organization of the PLC/RTU Database

Most system integrators blame low SCADA system communications performance on inadequate bandwidth. In some cases this analysis may be correct. However, in my career, I have found lack organization of the database in the PLC/RTU to be the largest contributor to poor system communications performance. I have seen PLC systems where 20+ polls of a single PLC by the SCADA application were necessary to acquire all of the PLC’s data. Put that PLC out on a half duplex 1200-baud radio system and the screen updates will probably slow to a crawl. Throw in 10 more PLCs on that same radio system and communications performance will get even worse, maybe even stalling out completely. And what if one of the PLC fails to communicate due to a problem? I hope you get the picture.

The best strategy I have found for organizing the SCADA database is when the RLL for the PLC is completed to execute the applications control functionality, add RLL rungs to organize the data necessary for the SCADA application in consecutive PLC registers. We can call the target PLC registers for SCADA data the scannable range. Most PLCs have the capability to do some sort of LET statements in their RRL syntax that allows the integrator to organize the data necessary for SCADA in consecutive registers in the PLCs memory. However, note that some older PLC/RTU devices may not allow the mapping of I/O points to another different location in memory. By doing this, your SCADA application may be able to acquire all necessary data in one or two scans vs. up to 20+ scans. Many PLCs SCADA protocols allow reading up to 100+ registers with a single poll. It takes much less time to acquire 100 resisters of data with a single poll vs., for example, 5 polls at 20 registers/poll. Take advantage of the protocols ability to handle long, multiple register packets.

Reason #2. Poor Historian Performance due to Lack of Organization of the Historical Database

How many times you have attempted to display a very long historical trend for some of your data only to have your historian stall or take an excessive amount of time to display the data? The usual reason for poor historian performance is excessive historical database file size(s). Most system integrators deal with this by configuring the system to keep only the last 7-30 days worth of data. In some cases this strategy may be OK but the ability to view important trends over longer periods of time may be lost. I have found that implementing a strategy to organize the historical database minimizes the amount of data archived. This can be accomplished without the loss of transients or important trends in the data and should allow for retention of data for a much longer period of time.

This issue may also cause your entire system performance to suffer as the SCADA application slows when updating an excessively sized historical database.

Historical Trending-Rules for Success

The best strategy I have found for organizing the SCADA historical database is following the rules below:

  • Know your data. Know what’s important to the end-user and make sure the historical is archived for those points. Also know what’s important to the maintenance technicians for the system and make sure the historical is archived for those points also.
  • Leave the historical trends turned off for the remainings points. If no one needs the data, be diligent about turning the point’s historical data collection to an "off" condition.
  • Use dead bands to minimize the amount of data collected. However, it is important that you do not set the dead band so loose that important transients are not captured. Again it is important to know your data to determine which points are the points where important transients may occur.
  • If your SCADA package has the ability to archive some of the older historical, yet still make it available at runtime, take advantage of that feature. The smaller the set of actively managed and updated historical data, the faster the updates.
For more information on this subject, click here. For information on how NetSCADA deals with this issue, click here.

Reason #3. Poor Organization and Layout of the Graphical User Interface and Process Database.

Many end users give their system integrators little guidance about how to lay out their SCADA graphic pages and what data to include on the pages. As a result, the system integrator develops the graphic pages with the data he thinks the end users need. Where it really gets confusing is when two or more distinct work groups (such as Operations and Engineering) both utilize the same pages and each group likes the data displayed in a certain way. As a result, the pages can become cluttered with too much data making it difficult to find the needed information on a page.

Graphic Pages-Rules for Success

The best strategy I have found for developing truly user friendly SCADA Graphic Pages is following the rules below:
  • 1. Keep it simple and uncluttered. Know what’s important to the end-user and make sure the data is on the page. If there is too much information for one page, create a second page and make a page link to access the next page. Resist the temptation to put too much data on a single page. Instead, add a page and move related data to the new page. Make it easy to access the additional page(s). Put the most frequently used info on the first page and less useful on the added pages.
  • 2. Use a consistent approach. Make page links to access the next page in a consistent manner. Locate them in the same place on a page for each graphics page in an application. Use consistent images in a consistent manner. Operators may be confused if two identical vessels or machines are represented by two different graphic images on different pages.
  • 3. Use animation sparingly. Excessive use of animation may distract operators from important information on a graphics page. Use animation selectively where it helps to indicate process flow or machine operation. Again, be consistent in the presentation of animated graphic pages to minimize the amount of data collected.
  • 4. Get them "talking on the same page". As I mentioned above, there is a big challenge when two or more distinct work groups (such as Operations and Engineering) both want to utilize the same graphic pages and each group likes the data displayed in a certain way. Be careful with this issue as pages may become cluttered with too much data making it difficult to find the needed information on a page in an effort to satisfy each groups need's. Resist the temptation to create separate pages for each group, if at all possible. Instead work with each group in defining what data is really important to each group and display that info on pages that will be used by both work groups. Data less important to Operations but important to Engineering could be located on an Engineering page and vice-versa. Keep in mind that when you get both work groups looking at the same data presented in a consistent manner, you will have achieved the objective of getting them "talking on the same page".
Read more reasons on NetSCADA site


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