A Supervisory Control And Data Acquisition (SCADA) system is a series  of computer programs that can be used to: acquire data from a process; graphically display the current and past states of process variables through several computer screens and printed reports; and to control the process. The PAC SCADA System was created as a suite of programs, written in LabVIEW, to provide SCADA functions. LabVIEW is a registered trademark of National Instruments, Austin, TX.

A real time data base (RTDB) is included within the PAC SCADA System to provide other system programs access to data (value, alarm status, etc.) that represent the current state of external process sensors and actuators. The external devices are accessed through some kind of I/O hardware like National Instruments I/O hardware, PLC, GPIB, serial port or image acquisition hardware. The SCADA RTDB subsystem insulates other programs in the computer from the task of directly accessing the I/O devices. It provides the means to access current data from external devices by identifying the devices by a simple ASCII string (called a Tag Name). Arrays of past Tag data can also be accessed from the Historian subsystem for any Tag that was archived.

The SCADA System consists of programs that provide capabilities in the following areas.

1)  Real Time Data Base The System can service a maximum of between 1000 and 5000 Tags. The number that can be reasonably serviced depends on the computer power available, the rate Tags are scanned into the RTDB, and other active computer processes. The following functions are configurable for each Tag: conversion from source units (e.g. voltage) to engineering units, alarm scanning and message generation, filtering (low pass, moving average and median), and historical data storage. All RTDB configuration information is stored in a tab delimited text file that can be read and modified by a spreadsheet or word processing program. RTDB Tag value data can typically be retrieved at a rate of about 10,000 Tags/second.

2) Tag Editor  Each Tag is defined and configured by using the Tag Editor program. The user enters configuration parameters such as Tag name, type, source, conversion factors, filter factors, history factors, initial value, engineering units, alarm parameters, descriptive text information and detailed source address information (i.e. hardware address parameters). All Tag definition data is stored in tab delimited text files.

3) Graphical Display Editor Process information can be displayed by the SCADA system in a dynamic graphical form to show the current and past states of the process being monitored and controlled. The LabVIEW graphic editor is used to build graphical and trend displays.

4) Data Historian Each Tag can be configured, using the Tag Editor, for automatic historical data storage at regular intervals. Tag data is compressed using regression analysis to fit value/time data to quadratic polynomials. The maximum absolute error for each Tag's fit is specified during the configuration process. Data compression ratios of between 10 and 40 can typically be achieved. For example, 24 hours of temperature data could use less than 1kb of disk space.  One of our users archived 2 years of 31 temperature data sensors , (sampled continuously at once per minute),  in a 13 MB directory structure.

5) Tag Trend and Parametric Graphs Programs are provided that retrieve archived data and display it in a LabVIEW graph. One program illustrates the display of time based trending of multiple Tags and another plots one Tag versus another with time as the common parameter.  Tag data can easily be retrieved as individual samples or as arrays of averaged data.  For example, one could display 52 weeks of daily averages for any Tag in the system.

6) Automatic Tag Report Generator The automatic report generator program steps the user through the process of specifying a report header, selecting Tags for the report, selecting the report start and end times and selecting the size of the time blocks used for averaging Tag value data for the report. The report data is retrieved from the Data Historian, manipulated and stored in a tab delimited text file for compatibility with a spreadsheet or word processing program.

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