How to homing the A-axis again with gcode?

Hello, I'm new here. I'm a bit of a novice. Sorry if I put the question in the wrong place.

I'm making a 4-axis swiss lathe. I've never had any mechanical problems. But I'm having some trouble with linuxcnc, I want you to help me.

I have a mesa 7i96s card. I control the spindle with step/dir and I want to use it both for turning and as an a-axis. I'm homing the a-axis, no problem.
I want to do this; I want the a-axis to find the ahome switch again using g28 in the program. The a-axis should rotate until it finds the switch. How do I reset the joint.3.homing pin with g-code? How can it homing again with g-code?


 
 
 

Thank you. I will check and try.

I want to ask you a question. How can I use the M19 command with Mesa 7i96s?

Can I call the M19 gcode and position the spindle?