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Homework Help: Logical diode circuit description

  1. May 8, 2012 #1
    1. The problem statement, all variables and given/known data
    i.imgur . com/tjtAC.png (Sorry! It's fine I promise!!)

    I was given this circuit and I need to know what exactly what kind of circuit it is. i.e. what kind of logical operation it performs. [itex]V_o[/itex] is the output. IN1, 2, 3 are the inputs. I need to know the states of conduction in each diode for every combination of the inputs. A logic LO input can be taken as 0v and HI can be taken as [itex]V_{cc}[/itex]

    2. Relevant equations

    3. The attempt at a solution
    I know that the left hand side of the circuit looks like it's going to be an AND gate. I understand the reversed diodes and semi-understand the pull up resistor. But I have NO idea why there are two diodes in series, and I don't know why there is a pull down resistor. Help would be much appreciated!
    Last edited: May 8, 2012
  2. jcsd
  3. May 9, 2012 #2


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    Staff: Mentor

    I haven't scrutinized this closely. But a good logic design would allow you to cascade an unlimited number of elements and not have it undergo a progressive change in logic voltages. That is, if logic 0 crept up as you progressed through one gate to another, you might find after passing through 7 or 8 gates instead of logic 0 being 0V it might have gone up in steps to become 5V.
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