Solution

The complete Step 7 Lite system is too large to be presented in its entirety for this submission. Instead, we will focus on one critically important piece.

A significant part of the process of programming an automation solution involves interactions between the programming system—the Step 7 Lite IDE in this case—and the particular configuration of hardware PLC components employed in the application. In order to make full use of all features in the hardware,  the IDE must know what modules are incorporated in the equipment and how they are interconnected. The process of setting up the IDE to conform to the PLC hardware in use is called hardware configuration. The IDE must be manually configured for the target PLC hardware because the actual hardware may be at a separate location or may not even exist when programming is begun. In practice, it is a surprisingly complicated and potentially error-prone part of the process. Because of this complexity, hardware configuration was a prime concern in designing the new system, especially since it  targeted less experienced programmers building applications of limited sophistication on relatively simple components.

The design of the Step 7 Lite support for the hardware configuration process is best understood within the context of the larger system. To promote productivity by PLC programmers, STep 7 Lite uses a flexible, multilevel navigation scheme offering simple, rapid access to any and all relevant views of the PLC application under development and direct switching among the various tasks of PLC programming (Figure 2). A project navigator on the left gives access to sets of related views and task contexts, which are accessible through tabs toward the bottom of the screen. This scheme also minimizes window-management overhead, such as rearranging and resizing overlapping windows.

Figure 2 - Overview of STEP 7 Lite with hardware configuration.
(Click here or on picture to see enlarged view.)

Recognizable physical and symbolic representations of actual equipment are used where appropriate to speed learning and reduce the probability of errors,. This is not the silly or gratuitous use of “real-world” metaphor, as in Microsoft’s infamous “Clippy” Office Assistant, but selected and carefully reasoned use of direct correspondence between the IDE and actual physical elements. For example, the control panel in the upper left of Figure 2 mirrors the appearance and behavior of the actual front panel on Siemens CPU modules.

A novel performance-support feature employed throughout the Step 7 Lite system is the “cascading” tool tip (e.g., lower left of Figure 2). Ordinary Windows-style tool tips provide only the briefest of messages. These may be adequate for simple identification of elements on a user interface, but often do not provide enough help or guidance for beginners. Cascading tool tips provide an added, more detailed or more advanced comment after an additional short delay. In Step 7 Lite, this secondary tip usually also includes an additional link into an appropriate entry in the main online help system.

Because configuring the hardware is required at the start, a new project begins with the Hardware Configuration view showing, as in Figure 2. Hardware configuration can also be revisited by the PLC programmer as needed at any time.

Performer tasks

The hardware configuration process involves several distinct but interrelated subtasks. The modules to be included in the actual hardware must be selected from among the available modules in the three series supported by Step 7 Lite. These include power supplies, CPUs, analog input and output, digital input and output, and specialized interface modules. In the actual equipment, the modules plug into specialized racks. The corresponding software representations within Step 7 Lite must be similarly located and interconnected. In addition, hardware modules have programmable settings or parameters whose values must be set in order to make use of the various features of the hardware.

The hardware racks are represented graphically to the left of the Hardware Configuration pane, with a tabular format provided in the upper right. The tabular display presents details regarding each included module while the graphic display corresponds to the physical arrangement and appearance of modules within racks. The two views are synchronized, allowing the user to manipulateor review the configuration within whichever view is most convenient or best fits their style of interaction. A graphic element (the blue line, Figure 3) connects the two views, thus enhancing the visual correspondence and facilitating interpretation by the user. This visual element, which follows the user’s actions dynamically (see the animated screen recording), reduces the chances of the user accidentally manipulating the wrong component.

In the lower right is a catalog of available components from which the hardware configuration can be constructed. The terms and abbreviations—such as PS, CPU and so on—are the familiar jargon of PLC programmers and Siemens users; the order of the catalog tabs reflects a natural but not required order for task performance. The closed cover of the catalog allows selecting the component series of interest and provides a “starting hint” for the first-time user. (A Windows XP-style one-time “balloon help” could be an effective substitute in future versions.)

Inside the catalog (see Figure 3), the user has full control over the level of detail in presented information. Pop-up “tool tips” provide additional information when the user pauses over a catalog entry.

Figure 3 - Catalog shown open.
(Click here or on picture to see enlarged view.)

Components can be inserted, copied, deleted, or moved within either the graphical or tabular view using any applicable standard Windows interaction idiom, such as drag-and-drop, copy-and-paste, target selection and double-clicking. However, the actual placement and interconnection of modules in real equipment is restricted by complex constraints, such as power supplies can only work in the first slot of a rack, multiple racks must be interconnected by certain interface modules that can only go in slot 3, etc. Step 7 Lite understands all these rules as well as the functions and characteristics of all supported modules, providing direct visual guidance to users. For example, when a module is dragged within the graphical rack area, slots  change appearance (“dynamic affordances and constraints”) to indicate where the particular component can or cannot be inserted (see Figure 4). This supports correct novice performance without interfering with highly experienced programmers who already know all these rules. In any case, incorrect operation is almost completely prevented. The system is also smart enough to allow expert users to rapidly and correctly accomplish a series of insertions in successive slots just by double-clicking or pressing the <Enter> key on a selected module in the catalog.

Figure 4 - Module insertion task.
(Click here or on picture to see enlarged view.)

To see and set parameters, modules can be “opened” by double-clicking or by selection and then clicking on the Module Parameter button (supporting expert and novice performance respectively). The scrolling control surface of the parameterization dialog (Figure 5) supports a single navigation and interaction scheme for any set of parameters, which can range from a single parameter to hundreds and come in many different formats. User errors in parameterization are highlighted in place and explained in a highlighted message bar that rolls down from the lower edge of the dialog box. Thus feedback to the user is provided in context without overlaying or interrupting the visual context.

Figure 5 - Mudule parameterization.
(Click here or on picture to see enlarged view.)

To appreciate fully the dynamic nature of the various performance-centered features of Step 7 Lite, see the animated screen camera recording.