Most 3 channel encoders are quadrature output (2 channels) and an index channel.
The quadrature output (channel A and channel B) have the following output (CCW):
A 0011001100..
B 0110011001..
The CW output is the reverse:
A 0011001100..
B 1001100110..
These outputs are usually decoded by a quadrature decoder to give a "step" and "direction" output (or CLK and U/D signal for counters).
The index channel gives a single output pulse once a revolution and is generally used to give an accurate "home" position.
Mariss |