Hi Borge,
The laser TTL spec is usually <0.5volts for off and >2.0 for on. Providing it does not draw any appreciable current this input can be connected to the Z axis direction pin  on the BOB so that it turns the laser on for -Z moves and off for +Z moves. I disable the Z axis so that the laser focus is not disturbed and use the Z axis DRO scale factor to control the 'laser bite time' (which varies from material to material). If the laser operation is reversed then change the direction active low under ports and pins.
Although I have used this method quite successfully I now favour a simple shutter system to switch the laser beam on and off, again using the Z axis direction signal as I believe that longer laser life will be achieved by using a soft start when it is turned on from cold.

Tweakie.

There is some further information here :-

http://www.cooperman.talktalk.net/index_files/Page4549.htm

Hi Tweakie.CNC
Could you or anyone else tell me why when I use this method, the laser stays on only a flash? The laser does not stay on for the entire path, only turns on for an instant. Do you use some kind of monostable to deal with this?

Hi Boxin,

I have replied in the other thread https://www.machsupport.com/forum/index.php/topic,33309.msg237520.html#msg237520

Tweakie.