reverse polarity digital logic

Discussion in 'General Electronics Chat' started by relicmarks, Apr 22, 2008.

  1. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    What can cause the output to in a digital chip to have the reverse polarity when its not suppost too?

    Like its suppost to be +5 volts but its output is - 5 volts , what can cause this ? any examples please?

    What should i test for that would cause this?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    what chip is it? what power supply do you use? Maybe you measure the voltage with the probes of the voltmeter in reverse:)
     
  3. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    all types of digital logic chips

    If the input signal is pulsing from zero voltage to +5 volts and the output is reversed polarity when it shouldn't be what causes this please?
     
  4. mik3

    Senior Member

    Feb 4, 2008
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    are you sure you measure it in a correct way? I think black probe of the voltmeter is inserted in the red pin of the voltmeter and the red probe in the black.

    is your supply voltage from 0-5 V only?
     
  5. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    are you sure you measure it in a correct way? I think black probe of the voltmeter is inserted in the red pin of the voltmeter and the red probe in the black.

    Yes i am measuring the correct probe polarity


    is your supply voltage from 0-5 V only? yes
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    On your voltmeter/DMM, the black lead should be plugged into the COM jack, and the red lead plugged into the V/Ω/mA jack.

    Your black test lead should then be connected to the digital circuit's ground.

    About the only way you would see a negative voltage in a digital circuit would be if an output were driving an inductive load that had a damaged or missing reverse-EMF protection diode, or perhaps a switched capacitor network.

    If there are overdriven linear ICs such as op amps in the circuit, you might see phase reversal (ie: latch-up), but not polarity inversion - unless there is the aforementioned inductive load or switched capacitor network.
     
  7. flubbo

    Member

    Apr 21, 2008
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    The only digital logic I've ever seen going from 0 to -5 volts is Emitter Coupled Logic.

    What kind of chips are you using?
     
  8. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    where specific voltage levels applied to the inputs of a digital function cause corresponding voltage responses on the outputs


    Truth char using voltages:
    A B Y

    0 Volts 0V +VCC
    0V +VCC +VCC
    +VCC 0V +VCC
    +VCC +VCC 0Volts


    74HC86D chip " positive logic"

    Pin 1 is +VCC 5 volts
    Pin 2 is the input for the digital signal (toggling state) square waveform
    Pin 3 is the output

    The output is Positive polarity square waveforms

    74HC86D chip "Negative Logic"

    Pin 1 is "Ground"
    Pin 2 is the input for the digital signal (toggling states) square waveform
    Pin 3 is the output

    The output is Negative polarity square waveforms


    Questions:

    1.) How and why did the output convert from Positive logic to Negative logic?

    2.) How else can i convert Positive logic to Negative logic without using a inverter chip?
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Negative polarity doen´t mean -5V on the output.
     
  10. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Negative polarity doen´t mean -5V on the output.

    What does negative polarity mean then?
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    Since you don't have a -5 supply, the negative polarity must mean that the output is inverted. Like an AND gate outputs a high when the inputs are satisfied, and a NAND gate outputs a low when the inputs are satisfied.

    If you are actually seeing a negative output on a 74HCxx chip, then either you're using +5 as the measuring reference, or your o'scope is running with the INVERT switch active on that channel.
     
  12. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Since you don't have a -5 supply, the negative polarity must mean that the output is inverted

    1.) Whats the difference between TTL inverted VS TTL polarity?



    If you are actually seeing a negative output on a 74HCxx chip, then either you're using +5 as the measuring reference, or your o'scope is running with the INVERT switch active on that channel.

    Yes im actually seeing a negative output on the 74HCxx chip , the circuit does have a +5 supply and a -5 supply

    But the way the 74HCxx chip is reversing the polarity doesn't use the -5 supply at all, it just uses the ground and +5 volts on the inputs to reverse the polarity on the output

    I'm guessing since the circuit uses a bipolar power supply +5 ,ground, -5

    Then you can reverse the polarity on any digital logic chips output by changing the ground and +5 volts on the inputs to any logic chip ? so the output flips the polarity?
     
  13. mik3

    Senior Member

    Feb 4, 2008
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    Positive logic means the high is 1 and the low is 0 and negative logic means the high is the 0 and the low is the 1
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Oh, sorry, there should have been Neagtive logic, insted of negative polarity.

    Meaning the same as inverted logic (or even inverted polarity in this case), 5V instead of 0V and vice versa.
    Edit: I see I am a bit too late
     
  15. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Inverter= Positive logic means the high is 1 and the low is 0 and negative logic means the high is the 0 and the low is the 1

    This is what a inverter does in digital logic

    Negative logic is different on the output on the oscilloscope you will see the squarewaveform pulses -5 volts NOT +5 volts they will be flipped to the - negative polarity

    INput is +5 volts pulses (positive logic) converts to -5volts ( negative logic )

    How can you do this ?

    why would designers use Negative logic or Inverted logic?
     
  16. Caveman

    Active Member

    Apr 15, 2008
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    You are being extremely unclear in this thread. First of all, try using the quote button because it is difficult to tell what you are saying vs. what you are just repeating.

    Now, it sounds like what you want to do is to drive a digital logic chip in a funky way. From the point of view of the logic chip, low (logic 0) is when an input or an output is close to the V- input (pin 7). And high (logic 1) is when an input or output is close to the V+ input (pin 14).

    So if you want to drive the chip with -5V connected to pin 7 and GND(0V) connected to pin 14 of your 74HC86D, then the chip will call -5V = logic 0, and 0V(GND) = logic 1.

    If you connect between +5V and -5V, then you should never drive 0V in because you should never drive middle voltages. This ain't analog, it's digital so stick to high or low. +5V is logic 1 or high, and -5V is logic 0 or low.

    Got it?
     
  17. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Positive logic:
    1.) High= +5 volts
    2.) Low= ground zero volts

    Positive "inverter" logic
    1.) High= ground?
    2.) low= +5

    (Because it had a inverter after the logic chip) so it inverted the high and low states

    Negative Logic:
    1.) High= ground
    2.) Low= negative -5 volts

    Negative "inverter" logic
    1.) High= negative - 5 volts
    2.) Low= ground

    (Because it had a inverter after the logic chip) so it inverted the high and low states
     
  18. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    Bipolar Logic:
    1.) High is +5 volts
    2.) Low is -5 volts
    3.) No ground or middle voltage
     
  19. mik3

    Senior Member

    Feb 4, 2008
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    does your circuit has a dual power supply? i mean +5 and -5 V?

    Can you tell me what chip is the inverter?
     
  20. relicmarks

    Thread Starter Active Member

    Oct 13, 2006
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    does your circuit has a dual power supply? i mean +5 and -5 V? YES



    Can you tell me what chip is the inverter? there is no inverter

    They use the 74HC86D as a inverter or polarity converter

    It takes +5 volts input but outputs -5 volts so its a polarity converter
     
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