You can hook up a normally-closed push button to a spare endstop input (e.g. E1) and use the M581 command to cause that button to execute an emergency stop if you wish. But as stephenrc says, one that cuts power to the machine is better.
Edited 1 time(s). Last edit at 12/11/2016 04:56PM by dc42.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].