High speed head crash into bed

[sfcgeorge]

Mid way through a Cura USB print I noticed it wasn't coming out well so I clicked "Abort" in Cura, which for some reason rams the print head into the bed at high speed and didn't stop until I powered off. This of course ruined the PEI tape :(

I realise this is probably a Cura issue, perhaps because on Ultimakers the bed goes down rather than the gantry.

But surely the Printrboard firmware should disallow commands that send the print head below the minimium Z. It homes at the start of the print and does bed levelling, so it knows where the bed is, I don't see why you'd let it smash through that.