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Embedded Systems December 2000 Vol13_13

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5. a. m. Desired Temperature Measured Temperature ••• ••• -,,-,,-, ,= L"_' that it actually plays tapes. The buttons to control the tape may be marked PLAY, REWIND, FAST FORWARD, and STOP giving further information. The obsel-ver now knows that the device has three fu nctions, and a way of halting any of them. Conditioning has taught u that button can be pushed and that dials can be turned. But what if the device has a Ie s conventional control. Ajoy- stick indicates that it should be grabbed by having grooves to fit fin- gers on the shaft. If a user approaches a device controlled by a touchscreen, they may not immediately realize that the screen's surface is touch sensitive. Making the on-screen buttons three dimensional will hint to the user that they can be pressed down just like a mechanical button. If the front panel contains buttons and dials, it will be obvious to the user that the buttons can be pushed and the dials can be turned. But what do those buttons actually do? Labeling is a delicate art. Sometimes a button per- forms more than one action, and it requires two labels such as a STOP/ GO button. The double label looks awkward. POWER does a good job of replacing ON/ OFF, but such replacements are not always availabl e. If a dial is graduated, then naming the units may replace the needs to nanle the fu nction. A dial gl-ad uated in degrees Fahrenheit does not need to be marked FURNACE TEMPERA- TURE, assuming that there are no other temperatures settable on the furnace. One common fault in industrial ••• •• design is to place buttons in a regular pattern, in order to make the appear- ance of the device more symmetrical. They line them up like soldiers, each one looking like the next. This makes sense if each has a similar meaning, such as each one repre- senting a different TV channel. If the buttons perform se parate functions tlJ' to group them according to func- tion. Use bigger buttons for the more popular functions, and keep the rare- but-nasty [unctions out of the way. If ce rtain buttons, o r a number of choices are likely to be pressed in sequence, an-ange them in a left to right orderin g, sin ce thi is the way people read, and that is the way they wi ll scan a screen or display. See Figure 1 fo r an example. Arrows are even more useful if the path that we want the usel- to fo llow is not as nat- ural as left to right. Surface area As the number of features in a device grows, the design is likely to hide many of them. This is often driven by the industrial designers who want a simple form and mechanical engineers who want fewer parts; many of the less fre- quently used functions end up under a 60 DECEMBER 2000 Embedded Systems Programming menu, or reusing tile buttons already avai lable. Consider a telephone tllat can access voice mail. If you sti ll have to use the buttons with digits on them, there is no clue by looking at the device that it is capable of accessing voice mail. Do not be fooled in to tllinking tllat you are doing the user a favor by making it look like any otller telephone that he has used. The phone would be far easier to use if the buttons ENTER VOICE MAIL, NEXT MESSAGE, and DELETE MESSAGE were added. The phone may look more complex, but users not inte rest- ed in the voice mail options will simply ignore the extraneous buttons. The front panel of a device is made up of a number of controls and dis- plays. A device with more dials, but- tons, and di plays ha a greater urface area. In th is context, tI1e term surface area does not refer to physical size, but the number of controls available and the number of actions that can be per- formed upon tllem. The more of the user interface's functional ity tllat is vis- ible to tI1e user, the easier it will be to learn tI1e whole device, and tI1e more obvious it will be what the device is capable of doing. The idea tllat use rs will be scared of a device with too many controls is a my til. Mo t people drive cars witl1 dozens of controls avail- able to tI1e drive r. Users will resent many control only if tI1ere is no organization, and tile controls do not provide adequate indication of tlleir purpose. Some of tile worst designs are a result of taking a product tI1at has a fundamentally complex interface, and trying to deliv- er it tI1rough a imple front panel. Devices are difficult to use because they have too few buttons, not becau e they have too many, If your software has to interpret tI1e ame button in many different ways, depending on tI1e context, tI1is is a sign that you may be cramming too much control into one component. I call tllis multiplex- ing a control. An interface will be less usable if the user has to decide, based on the current context, whether but-

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