Actve High And Active Low Pins????

Discussion in 'General Electronics Chat' started by dogar sahab, Apr 4, 2008.

  1. dogar sahab

    Thread Starter Active Member

    Mar 15, 2008
    116
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    i am quite confused regarding the active high and low pins..please guide me through the whole step....how they work and please tell me the practical application of decoders....where are they practically used????
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Can you give an example of what you're talking about? Chip number and the pins in question? Thanks.
     
  3. dogar sahab

    Thread Starter Active Member

    Mar 15, 2008
    116
    0
    i am talkin about the input at ACTIVE HIGH or input at ACTIVE LOW...what does this mean???how do they work???in case of decoders,lets suppose,what does this mean that FOR PROPER WORKING OF DECODERS,THE ENBLE INPUTS MUST BE AT ACTIVE HIGH LEVELS????
     
  4. veritas

    Active Member

    Feb 7, 2008
    167
    0
    Read this:
    http://en.wikipedia.org/wiki/Active_low

    For instance, if you have an IC with an active LOW reset pin, you would need to keep it HIGH during use of it, and make it LOW when you wanted to reset.
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The best approach is to obtain the data sheet for the specific encoder you wish to use. It will indicate each pin's function and the logic level that satisfies the enable or selection.
     
  6. techroomt

    Senior Member

    May 19, 2004
    198
    1
    if an "active low" device's output is turned on (active), the output signal will be a logic low. and vice versa, if an "active high" device's output is turned on the output signal will be at a logic high level. realize also that active low and active high can apply to inputs as well. since you are likely familiar with active high devices, just imagine an inverter at the pin so the signal gets inverted.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Active High means that the output of the chip (or a condition in the chip) changes when the input is made high.

    Active Low means that the output of the chip (or a condition in the chip) changes when the input is made low.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,177
    1,799
    The terms are difficult to explain out of context. If you remember DeMorgan's rules you know that each kind of logic gate can be drawn in two different ways.
    Code ( (Unknown Language)):
    1.  
    2. An AND gate can be written or drawn as
    3.  
    4. Y  = A  &  B       or
    5.  
    6. Y* = A* |  B*
    7.  
    8. The two forms are completely and absolutely equivalent.
    9.  
    In the first case both inputs, and the output are "active high", but in the second case both inputs, and the output are "active low".

    In the case of an individual signal like RESET, changing the name slightly may indicate the active state. You might put a "not bar" over the whole name, or use an asterisk, or a dash-H or dash-L, or an explanation mark in front of the name. Names consisting of just alphanumeric characters are often active high, but you should not count on this because there just are no standards.
     
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