Setting up EMC2 for hobbing. 
As show in this video: http://www.youtube.com/user/blyndpew#p/a/f/0/ZhICrb0Tbn4 

The hardware aspects are fairly clearly shown in the video,
the only other things needed are a Pyvcp panel with a spinbox in it for tooth count 
(which should be wired to the invert.0.in pin in the posgui.hal file,
and these some extra lines in the main HAL file: .

presumes spindle encoder is encoder 00 and scale of -200 
with A Axis amplifier enable is par port pin 9
and stepgen 3 is A axis 

loadrt invert count=1	# no divide in hal, so use invert and scale
loadrt scale count=1 	# scale is used to divide the spindle command by the gear ratio		
loadrt mux2 count=1 	# the mux is used to store the offset as a sample-and-hold
loadrt ddt count=1 	# used for change detection in the gear ratio to reset the axes
loadrt abs count=1 
loadrt comp count=1 	# convert non-zero to toggle a bit on change

addf invert.0	servo-thread #mult and invert are used to divide by the gear ratio
addf scale.0 	servo-thread
addf mux2.0 	servo-thread
addf ddt.0 	servo-thread
addf abs.0 	servo-thread
addf comp.0	servo-thread

# A-Axis
setp stepgen.03.step_type 0
setp stepgen.03.position-scale 	[AXIS_3]SCALE
setp stepgen.03.steplen 		[AXIS_3]STEPLEN
setp stepgen.03.stepspace 		[AXIS_3]STEPSPACE
setp stepgen.03.dirhold 		[AXIS_3]DIRHOLD
setp stepgen.03.dirsetup		[AXIS_3]DIRSETUP
setp stepgen.03.maxaccel 		[AXIS_3]STEPGEN_MAXACCEL
setp encoder.00.scale -200

# Electronic gearing of stepgen position to encoder position
net invratio invert.0.out => scale.0.gain	ddt.0.in 	# the ratio is linked in the postgui.hal to invert.0.in
net spindlepos encoder.00.position => scale.0.in 
net gearpos scale.0.out => stepgen.03.position-cmd
net aenable axis.3.amp-enable-out => stepgen.03.enable => parport.0.pin-09-out  # amplifier enable pin for axis 3 ( A axis) , set for parport pin 9 
# Reset Logic. Zeroes the encoder and stores the current stepgen position as an offset. 
# This is to prevent the gear rotating on tooth-count changes (often quite interminably)
net scale-change 		ddt.0.out => abs.0.in
net abs-scale-change	abs.0.out => comp.0.in1
setp comp.0.in0 0.01 	# this is looking for changes in 1/teeth as the raw value is not available until postgui.hal has run
net reset-spindles	comp.0.out => encoder.00.reset	 mux2.0.sel # this bit toggles high on a ratio change
net sample-hold	mux2.0.out => mux2.0.in0 	scale.0.offset # the mux is wired to itself as a floating point sample-and-hold
net gear-spindle-position stepgen.03.position-fb => mux2.0.in1 # wire the stepgen position to the sample and hold, and thence to the scale




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