Package supports simple DDE links from PLC to spreadsheet

A recurring need in process analysis is using data manipulation and statistical capabilities of spreadsheet packages on process data values. This includes charting, calculation of derived parameters, material balance tracking, or other functions. In any of these cases the ability to collect then import data from a controller into a spreadsheet is the preferred method in place of manual data collection followed by entry into the spreadsheet package.

The latter is a need for standalone systems, such as controllers with only simple operator interface devices connected or situations where a special data set is needed. The Dynamic Data Exchange (DDE) standard provides functionality to accomplish this transfer, guiding how applications share data.

For spreadsheets, DDE permits input of values directly into cells using a DDE link. Therefore, all that is then required is a DDE-compliant interface to the controller that will send data to the spreadsheet application.

Communication interfaces

For users of Allen-Bradley programmable controllers, Rockwell Automation (Mayfield Heights, O.) offers a communication package called RSLinx. This Microsoft Windows solution provides both DDE and OPC (Object linking and embedding for process control) interfaces from the supported Allen-Bradley devices to other software applications. The primary function of this package is communications between its Microsoft Windows-based Relay Ladder Logic (RLL) programming packages (RSLogix) and supported controllers. In support of that function, a limited function version called RSLinx Lite is provided with the programming packages. For the DDE and OPC communications interface to other packages the full RSLinx or the RSLinx Gateway product is required.

This package aids small system users by giving them a simple package for communicating with networked PLCs both for program support and data collection. This avoids the need to purchase, configure, and support a dedicated software package for the collection of process data when data needs are limited in quantity or in time duration.

Any user familiar with the set up of spreadsheets (Microsoft Excel, Lotus 1-2-3, etc.) can quickly establish the data file and begin using the charting and statistical features of the spreadsheet to analyze data. Creation of the needed Microsoft Visual Basic routines for building a database with the spreadsheet is documented within the Visual Basic help files.

Using the RSLinx package simplifies the task of establishing the DDE link. It requires no extensive training for configuration. The included help files provide adequate instructions such that most users can quickly and easily establish DDE communication from the controller side. Documentation is clear and simple—the package was implemented to support the DDE link function with minimal configuration steps.

One weakness of the software, however, is its inability to use tag names instead of only memory addresses for DDE link references. This requires the user to have a controller memory address listing available when configuring the links.

The current version of Rockwell Automation’s RSLinx Gateway Revision 2.00.97.30 works with Microsoft Excel 97.

For more information on RSLinx, visit www.controleng.com/info

Author Information

Contributing Editor, Tracy J. Coates P.E. is a consulting engineer at PCE Engineering, Johnson City, Tenn.

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