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> The Therac-25 was the first entirely software-controlled radiotherapy device. As that quote from Jacky above points out: most such systems use hardware interl
by ufmace 6y ago
> The Therac-25 was the first entirely software-controlled radiotherapy device. As that quote from Jacky above points out: most such systems use hardware interlocks to prevent the beam from firing when the targets are not properly configured. The Therac-25 did not.
This makes me think - there was only one developer there, I guess, who was doing everything in assembly. This software, and the process to produce it, must have been designed in the early days of their devices, when there would be expected to be hardware interlocks to prevent any of the really bad failure modes. I bet they never did change much of the software, or their procedures for developing, testing, qualifying, and releasing it in light of the change from relying on hardware interlocks to the quality of the software being the only thing preventing something terrible from happening.
- bluGill 6y agoThe software was working just fine for years before on earlier versions with the interlocks. They never checked to see how often or why the interlocks fired before removing them. Turns out those interlocks fired often because of the same bugs.
- brians 6y agoThey had two fuses, so they had a 2:1 safety margin! Just like the NASA managers who decided that 30% erosion in an O-ring designed for no erosion meant a 3:1 safety margin.
- Gare 6y agoA quote from the report: > Related problems were found in the Therac-20 software. These were not recognized until after the Therac-25 accidents because the Therac-20 included hardware safety interlocks and thus no injuries resulted. The safety fuses were occasionally blowing during the operation of Therac-20, but nobody asked why.
- baobabKoodaa 6y ago> The safety fuses were occasionally blowing during the operation of Therac-20, but nobody asked why. Have you tried turning it off and on again?