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Electronics => Projects, Designs, and Technical Stuff => Topic started by: rea5245 on April 06, 2017, 09:24:55 pm

Title: Q: Figure out pinout of a rotary encoder
Post by: rea5245 on April 06, 2017, 09:24:55 pm

I have a rotary encoder, labeled QEDS-7090. It came from the front panel of an HP logic analyzer. I'm trying to figure out the pinout, and I'm seeing some puzzling stuff.

I found a datasheet for another encoder that may or may not be compatible: ( If it's compatible, then the 5 pins are:
   1 - GND,   2 - n/c,  3 - Channel A,   4 - Vcc,   5 - Channel B

That's plausible. The QEDS-7090 has 5 pins and pin 2 is not connected when wired into the HP LA. I measured for diodes and saw that there's a diode going from pin 4 to 1, from pin 4 to 3, and from pin 4 to 5. There appear to be no other circuits.

I understand that the diode from 4 to 1 is probably an LED. And I guess it's plausible that would I see diodes from pin 4 to pins 3 and 5 (photodiodes or phototransistors? I dunno.)

But the weird thing is, if I connect +5V to pin 4, ground pin 1, and turn the wheel, I don't get any signals on pins 3 or 5 (checking with an o'scope). Weirder still is that if I connect +5V to pin 5 and turn the wheel, I don't get anything on pin 3 but I get a nice square wave on pin 4!

It's possible that the encoder is broken; I have no way of testing it. But I can't explain why I would see the square wave on pin 4.

Can anyone help me with figuring out this wiring?

Title: Re: Q: Figure out pinout of a rotary encoder
Post by: Andy Watson on April 07, 2017, 01:50:54 am
It probably has open-collector outputs - i.e. the outputs need to be pulled up to Vcc (or possibly down to Gnd) via a resistor. Try something like a 10k resistor between Vcc and an output. I guess that by applying power to one of the outputs you are seeing something happening on the Vcc line because the remainder of the circuit is forming a "load" for the photo-transistor(s). The fact that you see a square-wave from the Vcc suggests that you have been lucky in that stuffing power "up" an output pin has not (yet) released the magic smoke.

If you cannot find the datasheet, try to find a schematic of a device that uses the part and deduce how it should be used. This part appears to be common to many HP devices.

Title: Re: Q: Figure out pinout of a rotary encoder
Post by: rea5245 on April 11, 2017, 01:42:32 pm
If anyone stumbles upon this in the future: the pinout for the QEDS-7090 is: Channel 1, no connection, +5V, GND, Channel 2. The outputs are push-pull.

Matching the pins to the connector is straightforward since the "no connection" position has no wire.

- Bob

Title: Re: Q: Figure out pinout of a rotary encoder
Post by: CJay on April 11, 2017, 05:00:51 pm
That's immensely handy as I've had one of those kicking around my junk box for ages now.
Title: Re: Q: Figure out pinout of a rotary encoder
Post by: Ross Havens on October 06, 2019, 08:50:19 pm
I figured out how to use the HP/Agilent QEDS-7090 - had three of these in my electronics spare parts bin and never got around to using them. Only two of them worked.

However, These are very high quality encoders. That were pulled from an old hp switching power supply.

Anyway, I have them hook to an Arduino Uno and I found a simple sketch to test them. worked great, very accurate.  One full rotation is appears to be a count of 120 or -120 with this particular sketch.

pin 1 - output (red-white-black wire)  --> connected to pin 3
pin 2 - gnd (black wire)  --> connected to gnd
pin 3 - 5V --> connected to 5V
pin 4 - key NC --> OPEN
pin 5 - output (brown-white-black) --> connected to pin 4

Not sure which Pin1 or Pin 5 is chan A or B. But that really doesn't matter.  You see there is rotational information captured CCW - or CC + counts.

I hope this helps someone in the future.