Embedded Systems December 2000 Vol13_13

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MURPHY'S LAW . The system was designed with the rule that any change into or out of demon- stration mode would require a password. Such symmetry appeals to . engineers, but it was not the right design decision in this case. while the e nd-users wou ld nolo This lock kept users out of demonstration mode. In the example descr ibed pre- vio usly, the pro blem arose because the service technician had put the device in to d emo nstrati o n mode before re turning it to use. Once the problem was discovered by the anesthesiologist, the easy solu- tion was to switch back to normal opel-ating mode. Unfortunately, mak- ing that change also required a pass- word. The medical staff did not know the password and had to replace the monitor with a spare one. The system was designed with the rule that any change into or out of demonstration mode would require a password. Such symmetry appeals to engineers, but it was not the right design decision in this case. The demonstration mode required a lock-out. But once in demonstration mode a lock-in wasn't needed. In general, you want to make it difficu lt to change from a safe state to an unsafe state, but easy to move from an unsafe state to a safe one. Nexlmonth we will examine a rew more uses of inte rlocks, and how they can keep soft'ware e ngin eers and users out of tI-ouble. In the meantime, if you find this topic inte resting, it is well worth reading Donald orman 's The Design of Everyday Things.3 A small part of the book is devoted to inte rlocks. The rest discusses othe r design-for-usabil- ity issues. A small number of books are out there that make almost no mention of software, yet should be compulsory reading for software e ngineers. T hi s is o ne of th e m. Norman's examples a re almost all from the world of mec ha ni cal design , but the same principles ofte n apply to usable software, especially in the embedded world . While on the topic of books, if reade rs have a ny suggestions for other no n-soft- ware books th at benefit 'o ftware e ngineers, let me know abo ut them. [ will add any that look inte restin g to my reading lis t, and d iscuss the whole lo t in a late r column. The open source debate Bill Gatliff and r wrote a pair of articles last September where I took a fairly jaundiced view of the o pe n ource community, and the products emerg- ing from ilo 'l It received quite a reac- tion, and I apologize th at I did not have time to reply to everyone, but 1 have made all of tll e feedback avail- able online at l}hyslt:tw. Because many of the e-mails were lengthy, and many good points were made on both sides, I thought that my correspondents deserved a slightly wider audience. I ~vill try to make a habit of posting in te resting feedback on the Web site, so that any- one who either agrees with me o r who wants to put me straight will be able to see if anyone else was of a simi lar view. Of the response J got to the open source articl e, it all fell dOW1l fairly hard on one side or tlle other. One writer pointed out the virtues of the Latex typesetting system. I would have to agree that it is a fin e tool, and I used it quite a bit myself before the days of what-you-see-is-what-you-get ed i tors. However, Latex does not really enter the argument in the embedded com- munity, since it would never be some- thing that would ge t used on a n embedded system, though, admittedly, it might assist in the documentation of the project. The oth e r inte restin g piece of feedback was a reade r 's reactio n to how difficult it has become to down- 96 DECEMBER 2000 Embedded Systems Programming load Red Hat's CygWin distribution from their Web site. They have a vested interest in sell ing the distrib- ution o n CD rath e r than having it downloaded at no cost. Obviously it costs them mo ney to maintain the file server, and so this is a reason- able business direction , even if it does frustrate customers a bit. A more honest approach would be to charge a small fee for a faste r down- load , but Red Hat reali ze that such an approach would probably leave them open to criticism that they we re leavin g their free software phi- losophy. The real co ncern here is that business is dictating a limitation on their Web site. Try to figure out what can happen if you exte nd th is philosophy to the suppo rt side of Red Hat' business. If th ey have the opportunity to re lease a piece of code that is very readabl e, maintain- able, and so easy to port that users can do it for themselves, then they are going to hurt an important rev- enue stream. What wou ld your deci- sio n be? esp Nialll'vl:wphy has been writing soflwarp for user intelJaces and 'mpdiwl systems for 10 years. He is thl' author of Front Panel: Designing Software for Embedded User Interfaces. Mu' rphy 's training and consulting business is based in Galway, freland. Hf reached at References: 1. "Potentially life-threatening medical equipment failure," from comp.risks archive, July 1999. Available online at http://eatless. uk/Risks/20. 48. html#sub;6.1 2. Murphy, Niall, "Watchdog Timers," Embedded Systems Programming, November 2000, p. 112. 3. Norman, Donald A. The Design of Everyday Things. New York: Doubleday and Company, 1990. 4. Murphy, Niall, "Are Open Source and Innovation Compatible?" Embedded Systems Programming, September 2000, p.78. ' welcomes feedback and can be

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