Incremental Encoder, how does it work?

[frogeraie]

I intend to use an incremental encoder from Alps:
http://www.alps.com/WebObjects/catalog.woa...2E1220406.shtml
but I don't understand how this component works!
From the schematic shown on the site quoted above it seems to me that on pin A and B I should get alternatively, as the shaft is rotating, 0 and 5 V. However it is not what I get: it is always 5V. I am confused, I need some help. Thanks.

[Village Idiot]

Do you have pull-up resistors connected? Do you have terminal C grounded?

[kellys_eye]

In it's usual mode you have to 'decode' the A-B output to give 'direction' (CW or CCW) and 'pulses' to determine how far around the dial (so to speak) the knob is.



the schematic above shows an encoder connected to IC2 and IC1 (part of each).

IC2 pin 1 will be high/low depending on the direction (CW or CCW) and IC1 pin 3 will have the pulsed signal output.

[GPG]

^ Sets and resets floating IC2,3.
Here's another one.
http://sharesend.com/v8y8i

[frogeraie]

Yes, "Village Idiot" I have done exactly what is shown on the Alps' schematic I attached to my previous message, I've included two pull-up resistors on A and B and grounded C.
Thanks to kellys_eye and GPG for their schematics, but I do not have any problem decoding A and B output since I am using the code described here:
encoder
which works well with other encoders (Nubotics).
My question is: how does an incremental encoder work?
I thought it is similar to an old PC mouse with some kind of mask alternatively stopping or letting the light of some led reach 2 phototransistors as the knob is rotated. Then each phototransistor is periodically conducting, so I should have at A and B altenatively 0 or 5 V , which I don't!
Is my clumsy description of an encoder functioning completely wrong?
Can someone explain it to me correctly?
Thanks!

[kellys_eye]

Not 'alternately' at A and B - the waveform 'overlaps' so A and B can bth be at logic 1 at times.

The datasheet for the encoder should illustrate this principle.