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Question for the encoder-savvy folks on the forum:

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  • #16
    Originally posted by Sparky_NY View Post

    The drive the OP is using accepts single ended encoder inputs only and they must be over 3.5 volts. A encoder with a line driver will not work with the drive unless you rig up a line receiver also. (fought that battle and had to ditch the line drivers) Line drivers usually meet the RS422 spec and are used with a line receiver.
    This depends on the drive. Some drives will work just fine with single ended encoders hooked to the differential inputs while others wont.

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    • #17
      Its not a matter of single ended or differential, its a matter of the voltage levels of the RS422 standard. The drive model the OP listed has encoder input for velocity feedback that will not trigger at the voltage levels of a RS422 line driver. Some drives may have a RS422 line receiver in the drive, such is not the case with the BE25A20AC. The OP listed the drive he intends to use, other drives specs are irrelevant. The drive only has single ended encoder input for velocity feedback purposes and will not trigger if driven by a RS422 line driver. I learned this lesson the hard way.
      Last edited by Sparky_NY; 11-13-2016, 04:23 PM.

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      • #18
        I have driven a bunch of servo drives with single ended inputs with generic differential encoders. They usually output about 3.5v which is enough to satisfy the TTL single ended inputs I have used.

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        • #19
          Originally posted by macona View Post
          I have driven a bunch of servo drives with single ended inputs with generic differential encoders. They usually output about 3.5v which is enough to satisfy the TTL single ended inputs I have used.
          You were lucky then, if they have a RS422 standard line driver the loaded output voltage can be as low as 2V. Look at the RS422 spec, which is the standard for encoder line drivers. http://www.rs485.com/rs485spec.html

          Again, I am speaking specifically of the BE2520AC drive the OP plans on using, if the encoder input voltage gets near 3V it will not function. This voltage CAN go as low as 2V if the encoder has a RS422 line driver. For some reason you insist on referring to generalities when the OP has a specific drive he plans on using. The RS422 spec is also quite clear, it CAN be 3.5V as you experienced but it is not GUARANTEED, the spec is as low as 2V loaded. I have personally ran into this using the BE25a20ac drive with a encoder meeting the RS422 line driver spec which resulted in a runaway due to loss of velocity feedback. (last specific time I seen this was with a fanuc redcap, incremental encoder and BE25a20AC drive, I have also seen it with other encoders and the BE25a20ac drive, its best not to play roulette with specs)

          I won't comment further, the OP's questions have been answered.

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