Transmitting power and PWM signal through the same cable
I would suggest using separate wires for current return ("supply ground") and digital ground.  Connect them at each end via small-value resistor.  Suppose the wires in your cable are 0.5 ohms.  If you didn't separate out your supply and digital ground, then you'd lose two volts (out of 24) in your cable.  An 8% energy loss in the cable isn't wonderful, but it's not the worst thing in the world.  On the other hand, the ground level at the power consumer would be a volt higher than at the supply, meaning that a logic 1 which was output as 3.3 volts would be seen at the other end as only 2.3

If the main power supply was connected to the digital logic with a 1 ohm resistor on the ground side, there would be two paths via which ground currents could flow--direct through the supply return (0.5 ohms), or else through two ohm resistors and the data ground (2.5 ohms).  The downstream supply ground would be 0.83 volts above the source ground level, but the two digital grounds would be within 0.17 volts of each other (at the supply, digital ground would be 0.33 volts above supply ground; at the load, the digital ground would be 0.33 volts below supply ground).