# Homework Help: Logical diode circuit description

1. May 8, 2012

### pikminman

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. $V_o$ 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 $V_{cc}$

2. Relevant equations
n/a

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. May 9, 2012

### 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.