Custom controls can be laid out in positions that fit with the function performed. If there is an eject button on a DVD, it's intuitive to place it beside the slot through which the disk will emerge. If a GUI is the only means of controlling the device, all controls must appear on that display, which means that those controls are further from the related hardware.

Another disadvantage of the GUI is that space doesn't generally permit the important controls to be permanently visible. This may not be acceptable if the device is used in a situation where the user needs emergency access to certain controls, or where some monitored information must always be visible.

Custom controls can scale the user's action or exaggerate the process being controlled. A bigger steering wheel allows a driver finer control over the angle of the front wheels. The da Vinci surgical robot enables the surgeon to move his hand several centimeters to control the robot, which only moves several millimeters, allowing the surgeon a level of control that he could not achieve if he were holding the scalpel himself (Figure 1).

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Many embedded products get the best of both worlds by adding a graphics screen to support peripheral information, while the most important user dialog still takes place using custom controls. This combo is an attractive option. It allows little used modes, such as configuration modes, to be implemented using the GUI alone, while normal running uses both the GUI and the custom controls.

While the user is manipulating the custom controls, information related to the changes may be displayed on the graphics screen. For example, as the flow of water in a pipe is adjusted on a dial, a diagram depicting the tank could show the water level rise and fall as the user turns the dial up and down. Such graphics are particularly useful for novice users who are building up a conceptual model of how the system works.

Consumer items often find a good balance between graphical interfaces and custom controls. Many camcorders have a touchscreen interface (Figure 2), but they retain the record button and a spring-loaded lever to control zoom. Zooming with a touchscreen control would involve covering the picture that you're trying to film and wouldn't give the same level of physical control of the lens.

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Cell phones also keep vital buttons, such as the answer button and the volume control, outside of the graphical interface. While the iPhone achieved its minimalist look by having only one visible button--the Home button--it still allows calls to be made and answered on a button attached to the earphones, and the volume control buttons are discretely placed on the edge of the phone.