Though much attention is paid to the software-based portions of control systems, there remains a place for hardwired circuits in modern control practice. These collections of devices and wires serve to control many small applications and provide for safety systems and other functions.

Hardwired systems are typically defined using Ladder Logic Drawings produced by hand or on Computer-Aided-Drawing (CAD) packages. These methods permit the design of the system and provide electricians with instructions on how to wire the components.

A software package from CMH Software (Libby, Mont.) called "The Constructor" provides an alternative to these methods of creating electrical drawings. This tool combines the functionality of a CAD package for electrical drawing with circuit-simulation capabilities.

Its CAD functionality provides a library of standard electrical device symbols that can be placed in the circuit being designed. The standard device library includes a wide range of components including:

• Transformers
• Switches and pushbuttons
• Coils and contacts
• Lights and horns
• Motors

Each drawing may include mixtures of power sources. They can be single- or three-phase, ac or dc. Package power sources are simply devices to be placed as any other device. During placement, the user can apply names to each device by selecting the tag box and typing a short ID. Wiring interconnections are then made by clicking on the grid lines provided.

This part of the package is easy to learn and does not require CAD experience because the devices must be placed on grid locations; sizing is not an option since devices are drawn full scale. Contacts are easily assigned to specific coils though an "assignment" function. This function logically links the devices so when a coil is energized a contact functions.

This package differentiates itself from a standard CAD package in its ability to simulate the circuits created. In the upper corner of the screen display a switch icon is turned on/off by mouse click, putting the system in either design or test mode. In the test or "run" mode, the proposed design's logic may be proven and shorts found. The user can actuate individual switches while power flow is clearly indicated. If a short exists, the test/design switch trips and the system takes the user to the specific error.

The package offers a single-click method for zooming out to get a wider view of the system being designed. This is limited, however, as only two levels of display exist and users are limited to a full-size view with notations or a somewhat wider view with none. Method of changing between the compressed and uncompressed format is simple. However, the package could be improved if interim compression levels were available and notations stayed visible when a view is compressed.

The package has a very good symbol creation/editing function that allows users to expand the standard library to meet application requirements.

The first step in creating a symbol is to select a device class. Keep in mind that once a device class is selected and the symbol saved, there are no utilities for renaming or deleting this device. Therefore, if an error is made in selecting symbol type, the user is stuck with a specific function symbol that may not perform as desired.

The Constructor runs under Microsoft Windows 95/98 and NT. This review is based on version 3.0 operating under Windows 98.

For more information on The Constructor, visit www.controleng.com/freeinfo .