I imagine the whole thing started something like this: "Hardware interlocks are expensive. And if something goes wrong, the machine shuts down for days until we can get an onsite visit from a certified tech."

More like: "The software works fine on the 20, so let's get rid of that expensive interlocks." Plus the fact the hardware interlocks on the 20 saved their bacon countless times without anyone noticing.

Certain Ricoh laser printers also have a fail-deadly "feature". There are sensors in there that detect certain dangerous conditions like "paper jam" and "output tray full" that could result in a fire. Thing is, if you disconnect the switches, the printer thinks "everything is fine", instead of shutting down with an error message...

Well, if you actively bypass hardware interlocks, don't complain if the hardware burns down your house. Of course, it would be possible have those switches work the other way round, i.e. error by default, unless you sense good conditions. You could even use double throw switches connected two two inputs each, indicating error on one signal line and ok on the other one. That way, broken / disconnected switches could be detected. But how likely is a broken or disconnected switch compared to paper jam or output full? Unfortunately, the safe way needs two inputs per switch instead of just one. And that's why you get tiny SPST switches instead. Plus a footnote in the manual that tells you to keep the printer away from anything ignitable.

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

In reply to Re^6: Plotting a volcano eruption halfway across the globe by afoken
in thread Plotting a volcano eruption halfway across the globe by cavac

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