Embedded Systems December 2000 Vol13_13

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a. ID I!!. E. Standard icons for basic VCR control learn general rules that the user can apply to the whole interface. REWIND PLAY FAST FORWARD PAUSE STOP an input if the value is illegal. If the . numeric keypad is replaced with a dial, the dial can simply limit the range at its mechanical limits, avoid- ing the possibility of an illegal value being entered . In the last two examples, the system wa!> able to de tect that the input was not valid and took alternative action. What about the case where the user performs an action that is a valid input to the system, and then realizes that it was not the appropriate action. The undo command is a popular feature on many desktop applications. The problem with undo is that the user has to figure out how much wi ll get undone. Will a second undo go back further in to histol)', or will it redo the undone command. You cannot always afford to h it the undo button just to find out what will happen. Sometimes it is possible to provide a set of actions that allow any action taken to be reversed. If you can move a robot arm to the left, then make sure that you can move it to the right just as easily. The effort taken to undo the action is equal to the effort taken to perform the action in the first place- leading to a robust system. A tape recorder (and they do exist) with a fast forward button but no reverse button will be frustrating because the user will have to turn over the tape to undo the action taken when he wound forward too far. The effort to undo the action is greater than the effort taken to cause the action in the first place- leading to a less robust system. Similar situations arise with mode changes. If it takes 10 steps to get from mode A to mode B, but only one key press to get from mode B to mode A, then expect your users to be displeased if they enter mode A by accident. One acci- dental key press will take 10 actions to rectifY. This principle that actions and their opposites should involve similar effort is what Thimbleby calls commen- surate effort.! One case where you may want to make an exception to this is if mode B is a safer mode than mode A. You may then wish to make it quite difficult to get into the high risk mode, but easy to escape from it. This does not make the system easier to use but it does make it safer. In an embedded system, reversing some commands is simply impossible when the physical action taken by the device cannot be undone. You cannot unlaunch a rocke t! In these cases some confirmation is required. When you implement a confirmation there is a delicate balance between ease of use and certainty of intent. If you make the user type in a complex sequence every time they wish to perform an irreversible action then th ey are unlikely to do it by accident; however, ease of use will be reduced. If such actions are rare and the cost of an accident is high, the confirmation pro- cedure may be elaborate, such as two operators simultaneously pressing a button that is reserved for this pur- pose. The buttons cou ld be p laced far apart so that one user could not press both. The important thing is to avoid forcing the users to confirm so many details, so many times per day, that they confirm actions automatically without actually reconside ring the conseq uences. Consistency The overall consistency of the design is a property that makes it easier to 58 DECEMBER 2000 Embedded Systems Programming Consistency in the interface will be reflected in the software and vice versa, so the code can be a good place to detect inconsistency: "I can use th is bLinkLight O function everywhere except th is one place. Why? There may be something different abou t what the user sees as well." Consistency allows the user to develop general rules about how the interface works. When the user starts to explore a part of the interface that he has not previously used, it is these rules that he will depend on. Do not get lured into lowering your levels of consistency in the more advanced fea- tures, assuming that only expert users will be using those features, and that they will figure it out regardless. It is in these advanced features that consis- tency is of greatest value. The user may only rarely visit the more advanced features, and he will not want to learn them from scratch on each use. Affordance Affordance is the property that indi- cates how obvious a device's function is from its appearance. If I hand you a pair of scissors, you will notice that one end has proportions that fit com- fortably in the hand. The other end has sharp edges so you are unlikely to hold it there. The sharp edges rubbing off of each other indicate that it is meant to cut something. The scissors affords holding and cutting. Some th ings are obviously easy to use because you can see al l of the con- trols. A tape deck has a door that opens to reveal two spindles that a tape could be placed over. Often the spindles are visible th rough a glass panel in the door anc;l so are seen even if it does not occur to the observer to open the dool". The tape deck has the affordance of something that can hold a tape. Until you examine the controls you do not know if this device is only for copying, or rewinding tapes, or whether it can actually play the tape. If speakers are visible that will be a clue

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