Randy . . . .
Not sure how much info. you need , but it would most likely be using * two *
photosensors , with a single encoder disc . . . it would use one sensor for
the position and the other sensor is slightly offset ( in the encoder cut-out ).
The dual encoder disc would use two sensors and the offset would be
produced by the offset angle of the second disc.  The direction of movement
can be determined by the pulse that is seen first , while the disc travels.
Most setups rely on the difference in the phase of the two pulses , I like to
think of it as simular to a doppler shift , but mechanical .  If you need details
or designs . . . .please email offline . . . 

Regards . . . 
                        Steve Kelley

-----Original Message-----
From: Randy A. <Cnc002@AOL.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Friday, October 01, 1999 11:10 AM
Subject: Reading Quadrature Incremental Encoders


>Does anyone have information regarding how to read the Quadrature signals
>from an incremental encoder such as those used for feedback loops on servo
>motors on CNC machinery.  In fact I am working on a low cost controller for a
>single axis machine.
>
>Any and all suggestions will be GREATLY appreciated.
>
>Randy A.
>