Simple RLL programming tool

Most automation projects use some form of logic control that is typically implemented in a programmable logic controller (PLC). Actual form of the PLC varies from dedicated hardware to PC applications, available from a variety of vendors.

Regardless logic type, the ultimate goal is monitoring of input variables and then responding with output signals based upon a user-determined algorithm. This algorithm is developed for each application using an appropriate software tool for the platform selected. For many hardware platforms, the user can select from packages provided by either the hardware vendor or through generic third-party software tools for developing the application-specific program.

For the Allen-Bradley SLC 500 and MicroLogix families of PLC hardware, Rockwell Software (Milwaukee, Wis.) offers RSLogix 500 for programming and managing these controllers. This package has the advantage of being developed and supported by a sister company to the hardware supplier.

Staying with tradition

The programming aspect of this software relies on traditional Relay-Ladder-Logic (RLL) notation, while supporting configuration control through a well-defined file structure. RSLogix 500 allows experienced users of other packages to be productive without requiring specific product training. It also lets infrequent users become productive with little difficulty.

The RLL editor uses standard graphical representation for typical contacts and coils to depict logic, as it is not fully compliant to IEC 61131. Placement of all program items is easily performed by “point-and-click” action at the desired location then, by either typing the three-letter element name or a second “point-and-click” on the element toolbar. This aids the user in finding elements on the toolbar, which are grouped by type and selected by labeled tabs.

During program development, the user has the option of using actual I/O addresses or user (tag) names. When using user names, the link between them can be defined as entered or later in the process. Regardless, once a name is defined, all uses are coordinated off the main tag database.

In terms of functionality, RSLogix 500 software supports a range of processor capabilities in a straightforward manner, simply showing unsupported functions “grayed out” depending on the processor selected. This gives the user a common feel to the environment independent of the processor. In addition, the package has a very useful help function that provides specific instruction descriptions along with examples of the instructions.

File management tools included make program management simple, while allowing the user to structure the program as needed, including allocation of data types. The user is presented with base file(s) for creation of program code, but then has the option of creating others as subroutines if that offers advantages for a specific application. Likewise, users can elect to increase the size of the standard data files or to create additional files for data segregation.

Beyond the programming environment, RSLogix is also an excellent debugging tool. When connected to the target PLC and placed in the “online” mode, the package clearly indicates status of all discrete devices and provides a reasonable real-time display of numeric values. One inconvenient feature of the online mode is that all open ladder subroutines except for the main are closed on the transition to on-line.

RSLogix 500 operates under Microsoft Windows 95/98, as well as under the Microsoft NT platform. It is Windows 2000 ready and Windows ME compatible. This review is based upon revision 4.50.00 of RSLogix 500 operating under Windows 98. www.software.rockwell.com

For more information on RSLogix 500, circle 345 or go online at www.controleng.com/freeinfo .

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

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