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IndustryArena Forum > CNC Electronics > CNC Machine Related Electronics > differential vs. single-ended quadrature encoder?
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  1. #1
    Join Date
    Mar 2007
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    7

    differential vs. single-ended quadrature encoder?

    Perhaps this is a silly question, but can I connect a differential quadrature encoder (A, A', B, B') to a decoder for a single-ended quadrature (A, B) by simply leaving the complements (A', B') disconnected?

  2. #2
    Join Date
    Dec 2003
    Posts
    24216
    Quote Originally Posted by postvmvs View Post
    Perhaps this is a silly question,
    Not a silly question, in most cases, yes, it depends on what you mean by decoder? i.e. what are the requirements on the receiving end.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  3. #3
    Join Date
    Mar 2007
    Posts
    7
    what you mean by decoder?
    Specifically I was thinking of the quadrature encoder interface of the dsPIC30F.

  4. #4
    Join Date
    Dec 2003
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    24216
    Should be OK with just A B and common.
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  5. #5
    Join Date
    Dec 2012
    Posts
    192
    While connecting just the A and B signals is ok in most cases, differential signals are used to minimize the effects of electric noise (inductive coupling, capacitive coupling, etc.) in long electric lines as is usually the case in most machine tools.

    I would suggest using a DS9637A Dual Differential Line Receiver next to your PIC. This 8 pin chip turns a differential signal into a single ended one. By using it you minimize the possibility of counting noise pulses as axis position pulses.

    The spec sheet can be downloaded here:

    https://www.google.com/url?sa=t&rct=...,d.cWc&cad=rja

    The chip price should be around USD $2.00 and in actual CNC operation it can prove to be worth using it.

    Best of luck.

  6. #6
    Join Date
    Sep 2011
    Posts
    474
    Quote Originally Posted by Al_The_Man View Post
    Should be OK with just A B and common.
    Al.
    So where would this common go? To the ground? Ive been trying for days to get some differential encoders to work with drives that only accept A and B channels.

  7. #7
    Join Date
    Dec 2003
    Posts
    24216
    Normally the power common is what is termed ground or signal common, use either the A & B or the /A & /B pair.
    Leave the other two disconnected.
    If the drive does not provide the 5v power then when using a separate supply do not take the plus to the drive, just the ground (common).
    Al.
    CNC, Mechatronics Integration and Custom Machine Design

    “Logic will get you from A to B. Imagination will take you everywhere.”
    Albert E.

  8. #8
    Join Date
    Dec 2012
    Posts
    192
    I would only add one thing. You have to take care of the signals out of the encoder and to what inputs you connect them. If you reverse them in an actual system they might end decreasing the count instead of incrementing it and viceversa. In such case just interchange them.

    Best of luck.

  9. #9
    Join Date
    Feb 2007
    Posts
    962
    I will have to try the DS9637A (thanks BBM). I use the SN75157 on all my drives , it its getting hard to find and the DS9637 looks about 1/2 the cost. There is a small difference in pinout though.

    If your connecting a single ended encoder to a differential drive, the unused pin needs to be held at 1/2 the supply voltage if it has an internal pullup resistor. You can use a pull down resister the same value as the pullup.

    And POStV, on programming the DSPic30F, i've seen some open source code where they used an interrupt that happens every time there is encoder pulse. This really slows down the program. Its better to read the Encoder register with a timer (maybe 1000 times/sec) and just add the accumulated pulse count to your position variable.
    Manufacturer of CNC routers and Viper Servo Drives
    www.LarkenCNC.com and www.Viperservo.com

  10. #10
    Join Date
    Jun 2010
    Posts
    4252
    There's a problem here with differential encoder outputs which you need to know about. If the differential output conforms to RS422 it may very well not have enough voltage swing to drive the PIC interface. The solution is to use a compatible differential line receiver. I use the DS26LS 31 and DS26LS32A for this.

    Cheers
    Roger

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