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This is actually the forum for the grblshield, which is a spiffy single board solution that replaces the Reprap drivers and all those wires with a single 3 axis stepper controller that plugs right on top of your arduino. Simple, convenient, cost effective and lasts a long time.
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Seriously, I’d look over on the grbl github Issues list for an answer to this one. I have not used the end stops in grbl so I can’t help from personal experience. The grbl github is here: https://github.com/simen/grbl