controlling an add on laser to my gantry mill

I am really just using this post as a sounding board, I hope no-one minds.
I am attempting to add a 2w laser device to my mill. I have bolted it onto the front of my router and jury rigged it so it is kind of working.
My setup is using a Mesa 5i25 board to a gecko g540 driver, I have a pwmgen.00  at 50hz output going to the g540 then the VFD  output (pin8) goes to a Super-PID spindle control, this is all working well.
My thoughts are that I just put a mechanical switch into the pin 8 line to select between router or laser.
The GCode control of the spindle is via M3 commands which I guess is what all the convoluted spindle code that PncConf created does. so my thoughts were to modify my post processor to insert M3 S0 to turn the laser off at Z moves and M3 S30000 (max spindle speed) (which is set in the tool parameters) to turn it on prior to XY moves
I have seen others using analog.out and then be able to use m67/m68 to control on-off of laser, but I don't think that's possible with my setup - please correct me if I'm wrong as that is the point of this waffle.
The PP I have seen others use (askjerry) shows that by the time the Z axis has moved down greater than 1mm, Q value is at max anyway so would there be any point in doing it this way anyway?
So finally, is what I'm doing sensible or is there a better way with what I have? if I get it going I was going to invest in a higher power (cost) laser.
Hope this ramble makes sense, I can add my configs if required and any feedback as always, is appreciated
Eric
 

I also connected laser few watts to my cnc.
You can use M3 - for spindle and M4 (reverse direction) for laser.

If you want to control power of laser on-fly you need to use analog.out from g-code, becose in other situation, for example if you want to control power with Sxxx command - you will get very slow motion when you will change power of the laser.

Thanks for the response, can I ask how does using M4 help, I don't understand why I would need that, also with the hardware I have, there are no more outputs available unless I buy an adapter to use the second set of input/output pins on the Mesa 5125 (which I will do) then I should be able to use an analog.out with M67 code.

I do believe you are correct about slow response as the M3 Sxxxx command is done at every Z axis change and it is causing a slight break in the burn pattern which is obvious in lettering, if you or someone can assist with a better method I would appreciate it
Hope this makes sense.
Eric

Thanks Tommylight, I'll read with interest. My l-cheapo laser does have a PMW input but the 5i25 pwmgen output goes via a gecko g540 which converts it to an analog voltage level and this apparently I can only control via M3/M5 commands, It all works ok but it's not very satisfying and it gives small breaks in the burn when it turns the laser off/on between rapids.
Anyway Ill read this link and see if I can follow it.
When I'm happy with the results I will be buying a better quality laser.
Eric

Is it possible to flash the 5i25 with a 2nd pwm output?  One way to prevent burn-in at start and end is pwm Control in dummy axis.  so you can very easily control the time via feedrate. This kind of control is much faster. 

Thanks for the reply Chris, yes I am looking into that way also, I have just ordered a db25 to 26pin header for the 5i25 second port P2 and I could then program a second pwm.
I'm using the 5i25 because I get better timing with it, my PC with the parallel port only is really crappy and gives me really bad latency errors no matter what I try.
BTW it's not burn-in, I'm getting small non burn gaps at the end of a move.
I'm going to try giving Z axis greater acceleration, tomorrow (it's getting late here in OZ);
Eric

Thanks Tom, I had a quick scan of your files and at first glance your velocity and acceleration values were what jumped out at me, will try these numbers as a start and fiddle from there.
Eric

When you divide spindle and laser with M3 and M4, you will make software switch:
For example
"spindle.M.forward" you connect to servocdrive output,
"spindle.M.reverse" you connect to some laser pwm-enable pin.