On Sat, 04 Feb 2006 00:04:24 -0600, Richard J Kinch
wrote:

Don Foreman writes:

Gray code is relevant to absolute encoders, not to incremental
encoders. An absolute encoder will give correct position even if
the encoder was moved during a power-down situation.

Quadrature encoders are both incremental and absolute by virtue of the
index pulse, if you have cheap electronics, and you can tolerate homing in
the application.

A once-per-rev index pulse is still ambiguous if the encoder goes thru
more than one revolution in the travel range of the moving part, not
an uncommon situation. A rotary encoder might well be geared up for
higher resolution and accuracy.

"Absolute" generally connotes being able to sense absolute position
even after a power outage during which movement occurred one way or
another. The absolute encoder knows immediately where it is without
having to hunt for an index, a fiducial mark or a limit sense
condition.