[gnea/grbl-Mega Issue#151] Laser connection on Ramps1.4

This has probably been covered elseware, so apologies in advance.
I’m trying to get a working laser on my ramps board. It works fine on my 3018 machine.
I have three connections at the laser
Positive
Negative
PWM
I’ve connected positive and negative and intend to try and connect to D6 for 5v PWM.. Before I do this, changing code etc. I have this issue. When I power up the laser it is on all the time.
When I get a PWM connection will the laser then fire and rest as it should