diff --git a/README.md b/README.md
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@@ -3,7 +3,7 @@
 grblHAL has [many extensions](https://github.com/grblHAL/core/wiki) that may cause issues with some senders. As a workaround for these a [compile time option](https://github.com/grblHAL/core/wiki/Changes-from-grbl-1.1#workaround) has been added that disables extensions selectively. 
 
 __IMPORTANT!__ grblHAL defaults to normally closed \(NC\) switches for inputs, if none are connected when testing it is likely that the controller will start in alarm mode.  
-Temporarily short the Reset, E-Stop and Safety Door<sup>4</sup> inputs to ground or invert the corresponding inputs by setting `$14=73` to avoid that.  
+Temporarily short the Reset, E-Stop and Safety Door<sup>4</sup> inputs to ground or invert the corresponding inputs by setting `$14=73` to avoid that. Due to the internal pull up resistors on microcontroller the normally closed \(NC\) switches must be connected to GND and the coresponding input of the microcontroller.
 Please check out [this Wiki page](https://github.com/grblHAL/core/wiki/Changes-from-grbl-1.1) for additional important information.
 
 Windows users may try [ioSender](https://github.com/terjeio/Grbl-GCode-Sender), binary releases can be found [here](https://github.com/terjeio/Grbl-GCode-Sender/releases).