Digital Inputs, Pull Ups, Signal Width

Discussion in 'General Electronics Chat' started by ERobishaw, Jul 21, 2008.

  1. ERobishaw

    Thread Starter New Member

    Jul 21, 2008
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    Trying to create a product using various digital I/o boards and am having trouble understanding various issues.

    What does a pull up resistor do -- generally speaking when do I need one?


    The typical I/O device will have so many "general purpose I/O lines". If I want to read a signal from a device creating pulses, w/o using an interrupt driven I/o line, do I have to continually poll the I/o line for state? Meaning I can only determine if a pulse occurs if I'm polling during the pulse? And therefore, if a pulse has 5ms, I have to poll at-least that frequently?

    If a pulse device lists the output as having a "maximum sinking current of 100mA@36VDC" does that mean it's actually putting out 36VDC? And my digital input which operates 0-5V will not work?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    If a device has "totem pole" outputs, you generally do not need one.
    If a device has "open collector" outputs, it has no source of current; therefore it needs one. A suitably sized resistor connected to Vcc can supply that current.

     
  3. ERobishaw

    Thread Starter New Member

    Jul 21, 2008
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    Thanks.

    If I had an ouput device that produced a pulse width of 135ms but the input device was filtered to only read max 200ms... would it be possible to put capacitor inline to essentially widen the pulse width (maybe along with a resistor to bring the current back down)?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Well, you might do something like use a Schottky or switching diode and a small cap. If you poll the input and it's low, do nothing. If it's high, note that, then set the pin to output a zero to drain the charge from the cap, and then change it back to input.
     
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