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Help with signal termination — Parallax Forums

Help with signal termination

T ChapT Chap Posts: 4,223
edited 2015-05-23 09:56 in General Discussion
At one time I was looking for ways to reduce noise on signal lines and someone suggested this circuit. A 120 ohm and a .1 cap as shown. The signals are from an encoder and the components are next to the Propeller. I put this circuit on a new board but am getting a rounded off signal. If I remove one of the components then the RC effect goes away and the signal is correct. Any suggestions on this termination? There is already a Schmitt trigger before the Prop, this was an effort to properly terminate before the buffer. After looking at other examples online, this design is looking more like an AC termination method, but my encoder is square waves.
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Comments

  • kwinnkwinn Posts: 8,697
    edited 2015-05-23 05:36
    Not surprising that you get a rounded signal, that is an rc circuit after all. The circuit reminds me a bit of the 220/330 ohm terminations on high speed data and address buses used on old computers. The capacitors were there to reduce current consumption. I would suggest leaving out the capacitor and connecting the resistor directly to Vcc or Vss. If the encoder has an open collector output then both resistors would need to be connected to Vcc,
  • ercoerco Posts: 20,256
    edited 2015-05-23 06:47
    If you call your signal a "robot", xanadu can help you get it terminated.

    http://forums.parallax.com/showthread.php/161083-Liability-insurance-terminated-over-the-word-quot-Robotics-quot

    Sorry, it was just too obvious.
  • T ChapT Chap Posts: 4,223
    edited 2015-05-23 08:04
    I saw a nice lecture on termination and understand this much better now. He goes over the most popular methods. Basically this explains that you must calculate the RC to the real world conditions. In my case .1uf is way too big. I swapped out with 470pf and now the signals are perfect with the encoder speeds I am using. With the cap you do not have the signal reduction problems you get with the resistor only method. Although the dual diode appears to have it's advantages over all.

    https://www.youtube.com/watch?v=A4YH6JzRCfc

    40:00 Capacitor termination.
  • Heater.Heater. Posts: 21,230
    edited 2015-05-23 09:56
    T Chap,

    Thanks for the video link. A nice reminder of things long forgotten. Looks like I'll be sitting in on that whole course this evening...
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