> -----Original [email protected] > Let me add a bit to Doug Nix's comments. > > For the United States, the applicable standard is UL 1998. > http://ulstandardsinfonet.ul.com/scopes/scopes.asp?fn=1998.html > I strongly recommend implementing interlocks in hardware. > The UL group > that handles software and firmware testing is backlogged and > works slowly. > There will also be limits on what you can do to update > electronics and > firmware. The amount of testing required is very extensive > because there > are may types of single faults that could occur that may result in a > hazards. > > Search for "Therac 25" at Google or some other search engine > and you will > see one of the worst-case results of a reliance on silicon > for interlocks.
Indeed, the 'Therac' incident is a good place to start, but this particular failure mode was not from any hardware or software failure, but was from an incorrect user-interface design and control-function design. But you are very correct in the misplaced reliance on firmware. That is, it can be argued that there was not 'logical' error in the Therac code, but an user-interface design flaw that resulted in the machine applying a fatal radiation dose to the patient. And the 'seminal' recomendation of the investigation was to recommend a HARDWARE fix - interlocks to back up software control of both electron scanning and beam energy selection. Software that is safety critical must validate all code paths and quantify temporal determinism. Software that is safety critcal, for most industry devices and consumer goods, cannot be depended on as the only means of process interrupt and user protection; an electro-mechanical device must be ultimately relied on for protection and fail-safe operation. Previous posts have listed IEC 61508 and UL 1998 for guidance. I respectfully add DO-170, IEC R 24731, EN 50129, MIL-STD-882, and this link: http://www.softwaresafety.net/ luck to code-monkies, Brian - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
