So, my thoughts are - with the other end of the cable plugged into the computer(?) that any output on pin 17 (the X-Direction pin) will be driving the E-Stop input into Mach3. By moving the X-Axis one way you'll set the X-Dir pin to +5V, by moving the other direction you'll set the X-Dir pin to 0V. You might have to do this from the "Run" page/tab on Mach 3... then head to the diagnostic page to witness the setting of the pin-11 input (estop). Sounds obvious, but with moving the axis in one direction, pin 11 input should be active, and in the other direction pin 11 should be in-active. This is just intended to verify the behaviour of the PC/motherboard, and later to verify the binding of the pin to the e-stop input.