MPG

Use three BC337 (NPN) to switch Axis (1 Port toggle) and Feedrate (2 Ports mux):

 Mach3 Port ---c b e ---[2k2]--GND
                 |
                 _
                 2
                 k
                 2 
                 -
                 |
            Arduino-Port

Jogwheel

Mach3

A low : Pin 10

B high : Pin 11

VCC : PC 5V

GND : 0V

GND also to GND near P15

Arduino

Digital 2, 3, 6 : switching transistors (outputs, see source)

Digital 4,5 : Inputs for Buttons Axis and Feedrate to GND

Couple all GND, Arduino and Mach3 together !

The blue display is 1602 at I²C.

It works !