The LCD is at the end of a long, legacy LCD cable winding through and along the frame, carrying a single pathetic ground. It's astounding it works at all (the Prusa runs its signaling pretty fast in fact).
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I have crashed the MMU communication multiple times after touching the frame--- again, a single-ended communication line over a long cable. Both sides were still awake, but the UARTs got out of sync.
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Einsy itself? Not as likely. Lots of beefy internal ground plane there. But then there's the rPI that's often backpacking.... again, single ended GPIO with one ground over a single header connector....
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Ahem, correction, the GPIO is not used for command communication between the einsy and the pi. There's a dedicated hardware USART on both sides. But it makes no practical difference to what happens when a reference potential shifts.
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All the safety rules were designed for the use case of "enclosed in a metal frame". RF rules are a little more modern but still expect some passive shielding to do the work, eg, if you're in a plastic case, it has metal foil, plate or paint inside.
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....aaaand Prusa nee RepRap is none of these things. Raw circuit boards strapped to thoroughly nonconductive plastic with a ton of ungrounded metal interspersed. And ribbon cables.
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Anyway, if you want reliability... tie your Prusa's frame and V- to earth ground. Your display will thank you and you won't randomly get hung prints in the winter when the heat is on.
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...because expansion/contraction of PETG via heating and cooling during printing *all by itself* is enough to generate sufficient potential to crash your display. You don't even have to be there.
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Replying to @xiphmont
Expansion/contraction sounds a bit weird as a cause of electrical potentials. How about it being the liquid plastic acting sort of like a Van de Graaff generator as it passes through the nozzle?
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Replying to @NYarvin
I think what's actually happening there is the heating/cooling warping pulling the plastic up from the bed slightly in bursts. Certainly, popping a thin PETG part free with tweezers is a sure-fire way to bork the display/spontaneously reset the printer.
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Hmm. That could be already-charged plastic suddenly moving and generating a change in electric fields, or it could be plastic picking up a charge as it detatched.
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Replying to @NYarvin
I would expect parting to create a charge. I would also expect a sharper-edged metal implement that picks up the charge to be more effective than the plastic at concentrating the charge to the point it gets past the powdercoat.
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Replying to @xiphmont
You don't need conduction through the powdercoat; electric fields going through it are enough. Electrostatic charges are commonly kilovolts; even a slight echo of that will scramble signals.
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