On 6/21/2014 11:21 AM, Roland Jollivet wrote: > It is sometimes prudent to add additional hardware switches to doors or > arms to safeguard each machine from the other as well as the operators. I > would be loathe to rely on a serial interface. It's a bit like the scenario > of a software versus hardware Estop. Accidents happen when people change > configs, or run a different job without the right settings. However, they > are very unlikely to remove a hard-wired door switch.
The classic case of failure from relying solely on software for safety was the Therac 25. The models it replaced were already poorly designed but had mechanical switches and fuses that would pop if things went worng. The 25 had no major differences in its software, but eliminated all the mechanical interlocks and fuses so when an experienced operator got ahead of the software it could fire whatever amount of radiation it had ready, from next to zero to a lethal blast. On top of that it had a bug with a rollover timer and a trigger in the treatment chamber. If the operator pressed the fire button at the instant the timer rolled over it did the uncontrolled firing thing. One of the problems was a race condition between code that accepted operator input and code that setup the system to adjust the radiation dose. There was no check for proper settings before firing, the programmers just assumed no operator would be quick enough to input the parameters and hit the fire button before the machine was ready. --- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com ------------------------------------------------------------------------------ HPCC Systems Open Source Big Data Platform from LexisNexis Risk Solutions Find What Matters Most in Your Big Data with HPCC Systems Open Source. Fast. Scalable. Simple. Ideal for Dirty Data. Leverages Graph Analysis for Fast Processing & Easy Data Exploration http://p.sf.net/sfu/hpccsystems _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
