>> the real failure in the story of the Therac-25 from my understanding, is that it took far too long for incidents to be reported, investigated and fixed.
> the earlier (manually operated) version of the machine did have the same fault. But it also had a failsafe fuse that blew so the fault never materialized.
#1 virtue of electromechanical failsafes is that their conception, design, implementation, and failure modes tend to be orthogonal to those of the software. One of the biggest shortcomings of Swiss Cheese safety thinking is that you too-often end up using "neighbor slices from the same wheel of cheese".
#2 virtue of electromechanical failsafes is that running into them (the fuse blew, or whatever) is usually more difficult for humans to ignore. Or at least it's easier to create processes and do training that actually gets the errors reported up the chain. (Compared to software - where the worker bees all know you gotta "ignore, click 'OK', retry, reboot" all the time, if you actually want to get anything done):
But, sadly, electromechanical failsafes are far more expensive then "we'll just add some code to check that" optimism. And PHB's all know that picking up nickles in front of the steamroller is how you get to the C-suite.
When I worked at an industrial integrator, we had a hard requirement for hard-wired e-stop circuits run by safety relays separate from the PLC. Sometimes we had to deal with dangerous OEM equipment that had software interlocks, and the solution was usually just to power the entire offending device down when someone hit an e-stop or opened a guarding panel.
About a decade ago a rep from Videojet straight up lied to us about their 30W CO2 marking laser having a hardware interlock. We found out when - in true Therac-25 fashion - the laser kept triggering despite the external e-stop being active due to a bug in their HMI touch panel. No one noticed until it eventually burned through the lens cap. In reality the interlock was a separate kit, and they left it out to reduce the cost for their bid to the customer. That whole incident really soured my opinion of them and reminded me of just how bad software "safety" can get.
To be fair, reps don't really know anything deep about their product. They just parrot what they are told (or they wing it, which, i guess, can be lying). They are pushed to sell, and they will say anything to sell.
> And PHB's all know that picking up nickles in front of the steamroller is how you get to the C-suite.
Blaming it on PHB's is a mistake. There were no engineering classes in my degree program about failsafe design. I've known too many engineers who were insulted by my insinuations that their design had unacceptable failure modes. They thought they could write software that couldn't possibly fail. They'd also tell me that they could safely recover and continue executing a crashed program.
This is why I never, ever trust software switches to disable a microphone, software switches that disable disk writes, etc. The world is full of software bugs that enable overriding of their soft protections.
BTW, this is why airliners, despite their advanced computerized cockpit, still have an old fashioned turn-and-bank indicator that is independent of all that software.
> the earlier (manually operated) version of the machine did have the same fault. But it also had a failsafe fuse that blew so the fault never materialized.
#1 virtue of electromechanical failsafes is that their conception, design, implementation, and failure modes tend to be orthogonal to those of the software. One of the biggest shortcomings of Swiss Cheese safety thinking is that you too-often end up using "neighbor slices from the same wheel of cheese".
#2 virtue of electromechanical failsafes is that running into them (the fuse blew, or whatever) is usually more difficult for humans to ignore. Or at least it's easier to create processes and do training that actually gets the errors reported up the chain. (Compared to software - where the worker bees all know you gotta "ignore, click 'OK', retry, reboot" all the time, if you actually want to get anything done):
But, sadly, electromechanical failsafes are far more expensive then "we'll just add some code to check that" optimism. And PHB's all know that picking up nickles in front of the steamroller is how you get to the C-suite.