Therac-25 (was Gamma 11)

Jerome H. Fine wrote: <snip> The software was custom grown PDP-11 software from the AECL company. The problem was that the instrument could deliver a massive 25,000 rad blast over a 1 second interval due to the fact that the beam control mask was incorrectly positioned. A collimator with adjustible slits, bending magnets, and a blocker to safe the machine was how the basic machine set the dosage. Cryptic undocumented error messages, frequent malfunctions that had to be ignored in normal operation, and incorrect instructions to operators caused the malfunction to be unrecongnized. This was analyzed in great depth as it was the first computer caused malfunction that could be analyzed to study the responses of the people involved in the use of the machine. It is very educational to read this from time to time, when you think you have though of every systematic thing you should consider when designing a system. The software basically got the dosage setting displayed to the operator out of sync with the actual, and eventually you would deliver a zap of electron beam energy that felt like an electrical shock, plus hot coffee on your skin to a patient. Jim