Logic Hi Impedance Tri-State Measurement

Discussion in 'General Electronics Chat' started by DexterMccoy, Feb 23, 2014.

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  1. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    For Testing Logic Circuits

    How do you know when a Logic output is an Hi Impedance? or tri-state?

    I don't know when a logic output is shorted to ground or Open internally on the output or if it's a Hi Impedance tri-state output

    How can I tell the difference? if the output is shorted to ground or it's open internally?

    Some Logic Probes will detect Tri state hi impedance, but how do you know if the output is shorted to ground internally or it's blown output has open internally? because it would measure the same

    What else can i use to measure Tri state or hi impedance outputs for logic circuits?
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    a volt meter is usefull, also test for resistance. if shorted to ground, resistance would be very low, if just low output, voltage would probably be present, less than 1 volt. tristate outputs output normal levels for whatever family of logic they are till un enabled, then whatever else is on the line controls the voltage. if nothing else on the line, then output is a float voltage.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    1) Take a 100kΩ resistor (or even higher depending on the impedance you are checking) and connect it between the output pin and GND while the output is supposed to be logic high. If you get 0V at the output pin then the output is high-impedance/disabled/not_working.

    2) Connect the resistor between the output pin and Vcc or Vdd while the output is supposed to be logic low. If you get Vcc or Vdd then the output is high-impedance/disabled/not_working.

    This is also a way to test the drive capability of the output. In this case you would gradually reduce the resistor value until the output fails to meet the output level specs.
     
  4. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    If the logic chip is working, what voltage would I get?

    Isn't this going to overload the logic chip and damage it? by putting a 100K ohm resistor in parallel with the output of the logic chip?

    If the logic chip is working, what voltage would I get?

    Isn't this going to overload the logic chip and damage it? by putting a 100K ohm resistor in parallel with the output of the logic chip?
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Placing a 100kΩ load on the output of a logic gate will not damage it.
    Placing a 1kΩ load on the output will not damage it.
     
  6. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    What value would start to damage the output of a logic chip?
     
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