Sounds like you've got one of Denford's own DM turrets.
Is it like the diagram in this thread? - http://denfordata.com/bb/viewtopic.php?f=40&t=157
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Having had a quick look at the wiring diagram and the description in that thread, it seems to be a case of rotate turret clockwise until position matches, then immediately reverse the turret, at which point it should lock.
Depending on how fast the turret spins, you may have a problem, as with the 10Hz macro refresh rate, you have a 100-200ms reaction delay depending on timings. However I can't find any info on the timings required, so it'll have to be a case of try it and see.
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I've not done any Mach3 macro programming for a long time, so I'm hoping somebody will convert the following to the required Mach3 code-
Code:
While (TurretPosition <> CommandedPosition)
  ActivateTurretCW
EndWhile
  DeActivateTurretCW
  (a slight time delay may be needed here?)
  ActivateTurretCCW
While (TurretNotLocked)
  (Do Nothing)
EndWhile
DeActivateTurretCCW
You'll also want to add safety time outs at various places, as if the turret fails to lock, you don't want it to sit spinning endlessly, until you manually stop it running.
One option you do have, due to the turret using a VFD, is you could slow down the rotation to give Mach more time to respond.