A lot of SCADA systems involving telemetry, especially in the water/wastewater industry, have a minimal amount of data coming from each site. A wastewater collection system with 100 sites, for example, might have only 1CONTROL ENGINEERING China版权所有,000 to 2,000 tags. (These data points can be either an analog value like "level" or a digital value like "pump running.") As progressive system integrators have implemented more integrated control systemswww.cechina.cn, however, tag counts have risen significantly. Proper planning is required to build and successfully manage a system of this magnitude.
　　ParasynCONTROL ENGINEERING China版权所有, a system integrator based in Brisbane, Australia, has had a lot of experience with large SCADA systems. And, somewhat unusual for a system integrator, Parasyn is continuously performing R&D in new areas of SCADA technology. "SCADA systems aren't scalable out of the box; you have to plan with the final footprint in mindCONTROL ENGINEERING China版权所有," says Tony Poole, managing director of Parasyn. And the right place to startwww.cechina.cn, he says, is with the business outputs.
　　SCADA system requirements should include detailed specifications for the following:
　　Access times—for alarm annunciation and alarm acknowledgement;
　　Data resolution—e.g., for each data point, the kind of deadband or time resolution needed for later reporting or analysis;
　　Visualization tools—e.g., the number of data points on the graphical interface vs. the update time needed;
　　Reporting requirements—the kind of reports needed and how often they are to be run.
　　"Once we have this established, we can start to design the system and test each element to ensure that it functions correctly under real-life conditions," says Poole. "The elements are communications, the drivers, the SCADA (or HMI), and the historian. The methodology of how to manage the flow of data across all the platforms is the key to application performance and business success." Time series data and its relationship to the historian can be especially challenging.
　　Time stamps
　　An essential element of