Interconnecting OR and AND gates (wired OR connection and wired AND connection)
We make a connection of 2 AND gates and 1 OR gate, as shown on the picture.
If we analyze carefully the picture, we notice that one of the two AND gates outputs will be in logical "1" (high level), when the A and B inputs or the C and D inputs are in logical "1" (high level).
These outputs serve as inputs for the OR gate.
Analyzing the output of the OR gate and assuming that the values of the resistors are the same throughout the circuit, a voltage division has been created, then the output voltage will be approximately: Vout = (+V -Vd)/2
By replacing V and Vd with real values on the last formula we get: Vout = (5-0.7)/2 = 2.15 Volts. (We divide it by 2 because there are two resistors of equal value on the electric flow)
In the case in which all inputs (A, B, C, D) are on logical "1", the two resistors of the AND gates would be in parallel, and they would be in series with the OR gate resistor. This would provide an output voltage of 2.85 Volts.
This level is in the not allowed (prohibited) area for a logical "1".
If we put more gates in cascade the problem will be more serious, so this method is only used for simple gates.
Note: Vd = Diode voltage drop = 0.7 volts.
