2801 and reflection

Discussion in 'The Development Lab' started by charleskerr, May 17, 2012.

  1. charleskerr

    charleskerr New Elf

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    1st, from the data sheet:

    The Connection and Driving of Cascading Operation
    The transmit the data over long distance by cascading, the WS2801 integrates push-pull output stage with strong driving capability which enables the data and clock can be transmitted up to 6 meters at 2MHz clock frequency.
    To prevent the reflection, it is necessary to connect a 50Ω resistor at the data input or output port for impedance match.


    Now my questions:
    1. On my pixel controller, does this mean I should have a 50 ohm resistor from data to ground and from clock to ground to match up when I connect to the controller strings.
    2. If I am building my own strings, on each smart pixel, on my output, I should add a 50 ohm resistor on the clock and data lines to ground.

    Have I understood that correctly?
     
  2. David_AVD

    David_AVD Good news, everyone!

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    I don't believe any of the commercial WS2801 pixel strings or modules include the 50R resistors mentioned in the data sheet.
     
  3. Timon

    Timon Apprentice Elf

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    I'd say that this only comes into play for long lines. It may be a good idea to add them if the distance from the controller to the string is long or when you have a large gap between two sections of a string. This is easily tested with a scope.
     
  4. OP
    OP
    charleskerr

    charleskerr New Elf

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    Yea, I noticed that commercial strings don't have them. However, I am making my own strings, so I could add them to the boards I am making for the pixels.


    In addition, I am trying to determine if my controller needs them as well.




    So based on the the spec sheet, what do people think.
     
  5. Skunberg

    Skunberg Full Time Elf

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    It seems from what I've read in this hobby about pixels almost nobody uses termination. The few than do are on long runs acting abnormal. The 50 ohm resistor is the first detail I've seen of termination other than putting it at the end of the run to prevent reflection. Hopefully we'll get some data from someone that has needed to use them.
     
  6. OP
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    charleskerr

    charleskerr New Elf

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    Let me re ask the question differently. Based on the data sheet, is it saying that every Data in on the 2801 should have a 50 ohm resistor to ground on it?


    Should the last pixel in the string have a 50 ohm resistor on data out?


    Also, should my controller that feed this string, have a 50 ohm resistor to ground on the data going to the string?




    Based on the data sheet, should anything be on the clock line?
     
  7. AAH

    AAH I love blinky lights :) Community Project Designer

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    My thoughts are that as the pixels are typically 4" apart and each of them retransmit then it should really only be the 1st pixel that needs terminating. As i've never touched a pixel or looked at the 2801 data sheet I can't say for certain though. In a week or 2 I will start enjoying playing with 2801 pixels but until then I'm a complete novice.
     
  8. lithgowlights

    lithgowlights Senior Elf

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    Actually some of the higher quality ones do indeed include the 50 ohm resistor on the pixel themselves, as if it is in the spec sheet then it is there for a reason.

    Having said that in practice for our hobby the requirement is probably overkill as the pixels are close, in relatively low EMI areas, and not as critical as some used in commercial situations.
     
  9. OP
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    charleskerr

    charleskerr New Elf

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    So on the ones that do have them (per your reference), where are they? On the data lines on input, or output?


    Are there any on the clock line?


    I assume they are 50 ohm to ground.
     
  10. TimW

    TimW Full Time Elf

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    Hi Charles
    I doubt they are 50 ohms to ground. More likely 50ohms in series with the signal.
    Think of the resistor as dissipating the reflected energy (it has to go somewhere!)
    (I expect the cumulative current effect of all those 50ohm resistors to ground might not be good as well!)
    There has been a lot of discussion on terminating resistors into pixels from the controller ... so it has been an issue before.
    If you have them on the data line, I would imagine the clock lines are electrically very similar!
     

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