Question about behavior of decoders

Discussion in 'General Electronics Chat' started by gammaman, Mar 28, 2009.

  1. gammaman

    Thread Starter Member

    Feb 14, 2009
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    Is it true that when enable is 0 all of the outputs are 0?
    If so, then why is it that when you have 2 enables both equal to 0, all the outputs are high?
    What if you had decoders with enables of 0 and 1?

    The text book I have is not very clear and is very brief on the decoder. I have an exam next week and I just need to know a general rule for enable and decoders as it corresponds to the output.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    You have to read the datasheet for the particular decoder you want to use.
     
  3. gammaman

    Thread Starter Member

    Feb 14, 2009
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    I am in a first year course in digital circuits. I just need some general rules.

    For example: S1=0,S0=0:
    Say I have a circuit in which none of the outputs are being inverted. Now if I have 2 enables what happens when

    E0=0,E1=0? I think they all outputs are high?
    E0=1,E1=1?
    E0=0,E1=1?
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    I agree with kubeek. It is difficult to discuss the details of a digital function performed by a device in general term. It is easier when a particular part number is provided.

    hgmjr
     
  5. gammaman

    Thread Starter Member

    Feb 14, 2009
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    I don't mean to be rude, but if I am given a block diagram of a decoder on a test with one of the combination's I gave above, with no other information, how am i supposed to get the correct answer?
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
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    Can you post a typical diagram that you might encounter on your test? That would help us better answer your inquiry.

    hgmjr
     
  7. peajay

    Well-Known Member

    Dec 10, 2005
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    Is it true that when enable is 0 all of the outputs are 0?
    If so, then why is it that when you have 2 enables both equal to 0, all the outputs are high?
    What if you had decoders with enables of 0 and 1?


    What's making this a complicated question is that you're speaking in terms of digital 1s and 0s when you need to be speaking in terms of active or inactive inputs and outputs if you want to speak in generalities rather than speak about specific decoders.

    An enable input may be active-high or active-low and an output may be active-high or active-low. Whether each pin is a logical 1 or 0 depending on when which inputs are logical 1 or 0 depends on which inputs or outputs are active high and which inputs or outputs are active low.

    Take the 74HC138 for example: http://www.jameco.com/Jameco/Products/ProdDS/45330.pdf

    This chip has active-low outputs, and it has three enables, two of which are active low, and one which is active high. Additionally, the enables are configured so that all three must be active at once in order to enable the decoder, which doesn't necessarily have to be the case, although it is true of every decoder I've ever seen.

    So, for the 74HC138, unless pins 4 & 5 are low and pin 6 is high, or in other words, unless all three enables are active, then all outputs will be high, which is their inactive state. So for this particular decoder, when the inputs on pins 4, 5, and 6 are 0, 0 and 1 respectively, then all outputs will be 1 except for the selected output which will be 0, otherwise all outputs will be 1. However, if you speak of a different decoder, some of those 1s and 0s may become 0s and 1s instead.

    Exactly how you would know how to answer this question on your test is a good question. There are certainly any number of ways one could draw a symbol for a decoder. If it looks like something you're familiar with then that helps. If not, then, well, that sucks.

    Personally, I'd just bitch and moan if I were presented with such a question that lacked a truth-table for the decoder in question. I don't rely on the schematic symbols for these things since for approximately 10% of such drawings, the moron who drew the thing simply copy & pasted the symbol for something else and then failed to change a few details, and so you end up with symbols with random inaccuracies. Truth tables are much more reliable, with only a 1% error rate*, but you have to read them in the sense of "what was the person who wrote this table trying to say" rather than "what does this table say," just like if you were reading a paragraph someone wrote about the chip, you would try to look past problems with their grammar and see what it is that they're trying to say.

    * Yes, I'm making these numbers up, but who cares?
     
  8. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    There is no 'general rule' only specific rules per a chip's design. If you could post a picture of the diagram of the device drawing to be used perhaps we could give you better pointers.

    In general chip enables can be active high or active low signals, look to see it the input pin has a small bubble called an inversion sign (active low) or not (active high). If there is more then one enable then both enables have to be in their active state (they form a AND permissive) to free up the outputs. The voltage level that disabled outputs take on will vary with the specific chip used, could force all outputs low or high or go to high impedance output.

    Lefty
     
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