a. Considering the light from the lamp as the output, let's relate the whole circuit to either
positive logic or negative logic OR gate. We've already determined that it follows the rules of an OR
gate but how can we say that it represents one type of logic or another? Look at the voltage input and
consider the circuit operation again, first as a positive logic.
b. When both switches are open there is no voltage or 0 volts connected to the lamp and there is
no output-light. The two operating voltages then are +6 and 0 volts and, since positive logic requires
that the more positive voltage is the binary 1 or active signal, this circuit operates as positive logic
because we have a positive output (light) when we have a positive input.
By changing the operating voltage to the switches, we can easily show the same circuit as a
negative logic OR gate. When either switch (or both) is closed, the negative voltage lights the lamp.
Since the more negative voltage produces an output, the circuit follows the rules of a negative OR gate:
any one or more low inputs produces an output.
There is one other fact about the OR symbol that we should relate to the schematic. An OR gate
can have any number of input connections and still follow the rule that any one (or more) active input
produces an output.
Look at Figure 3-1 again and imagine more switches connected in parallel with A and B. Close
any one switch and the lamp still lights. It's still an OR gate no matter how many switches are
connected in parallel.
A functional circuit (diode or gate) that is used in many applications can be almost as simple as
the circuit in paragraph 1. Figure 3-2A shows a simple circuit that functions as a positive logic OR gate
and Figure 3-2B shows a similar circuit that functions as a negative logic OR gate.
a. For A, Figure 3-2, let's say the operating voltages are +6 and 0 volts. When either A or B is
connected to +6 volts, the associated diode is forward biased. Current flows from ground through the
resistor, through the diode to the voltage source (the input). Since almost all the resistance in the circuit
is the resistor, it will drop all of the voltage. Therefore, the voltage at the output will be +6 volts, the
same as the input.
b. It might be easier to see if you imagine each diode as an automatic switch. When zero volts
is connected to both inputs, the switches are open and the voltage below the resistor (0 volts) is the
output level. When +6 volts is applied to an input, the associated switch closes and the +6 volts is
connected straight through to the output.
Part B, Figure 3-2, shows a negative OR gate with 0 and -6 volts as the operating voltages. It is
exactly the same as part A except for the position of the diodes and the input voltages. When one or
more inputs is low (-6), the diode(s) is forward biased and the output is low (-6). The six volts are
dropped across resistor R1. When all inputs are high (binary 0) the output is high.