Optical quadrature rotary encoder - odd circuit or is it me?

diagram...

Are what you have shown as photodiodes really diodes with about a 0.5 to 0.7 forward drop or are they some sort of photo resistor?

It is normal to operate a photodiode with a back bias on them. If they are really photodiodes I can suggest that the full circuit looks like this:

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Drat! seamonkey is using proportional fonts and I can't see how to change it. I was going to do an ascii art for this but instead I'll have to use english

Photodiode 1: Cathode goes to +5V Anode goes to 100K pull down called signal A

Photodiode2: Cathode goes to 100K pull up called signal B Anode goes to ground

Signal A and signal B are likely to be two square waves with a 90 degree phase shift between them. Inverting a square wave won't change this fact so the circuit will still show rotation but it will indicate the reversed directions from a more normal circuit.

Photo diodes can operate either in a forward mode where they develop a current and voltage like a solar cell or they can be operated in the reversed mode, reversed biased, where they act as a photo sensitive resistor but generate no current of their own.

This schematic looks like the later, reversed biased mode. D1 cathode goes to +5 volts. The anode goes to an unknown circuit but likely an amplifier input with a resistance to ground. D1 is thus reverse biased and is a variable resistance to the amp input.

D3 is the inverse of this. Anode to ground, cathode likely to a resistor to

+5V and the input of a second amplifier. D3 is therefore also reversed biased but it's signal comes off of the cathode while D1's signal comes off of the anode.

In this way signal polarity of the two diodes is inverted, one goes up while the other goes down in a push-pull arrangement.

Without seeing all of the circuit where these diode connect its difficult to know if this is the right interpretation. Do you have the rest of the printer to dig deeper into the circuitry?

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I ran each of the signal outputs into a trace on my scope today. Sure enough, I get two logic-level pulse outputs, out of phase. Looks like there's some hidden magic in the bottom of the emitter-detector assembly that drives these outputs. I was assuming that they were individual photodiodes((or photoresistors), and I thought I'd have to source current on the signal wires - that's why I never bother to just hook them up to the scope... live and learn.

Thanks for the help!

Almost always, the detectors are phototransistors, with a pullup resistor on the collector. Phototransistors or photodarlingtons are cheaper to make (less active area required for a given output current) but slower than photodiodes. The LEDs are usually IR types, might be two in series.