Figure 3-12. Transistorized positive NAND circuit
a. The low on an input causes the associated diode to be forward biased because the diodes are
connected with their cathodes toward the input. The diode conducts and the low is coupled to the base
of Q1, cutting it off. This allows the output to rise to a high. Any low into this circuit causes a high out.
b. When all inputs are high, no input diode conducts. The positive supply voltage on R1 biases
Q1 in conduction, causing a low output. This is shown in line 4 of the truth table. The schematic then
follows the rules of a NAND gate. As long as the inputs are NOT ANDed, the output is high.
Learning Event 4:
COMPARE A NEGATIVE AND POSITIVE LOGIC CIRCUIT
1.
While reading the preceding paragraphs, you may have sensed that there was something familiar
about some of the circuits and truth tables. If you did, the feeling was justified, there are some
redundancies.
a. A circuit that performs a particular function in positive logic can be the same as a circuit
performing a different function in negative logic. In fact, they can be identical. And yet, they are
represented by different symbols because the symbol only explains the circuit's function in some
situation. Put the circuit in a different function and it would be represented by a different symbol. The
symbol, after all, only tells you what the circuit will do under different input conditions, not how the
circuit is physically constructed.
b. Now let's find out how this is possible. Look at the circuit in Figure 3-13 and be sure to
notice the title. On a separate paper complete the truth table (part B) and draw a symbol to represent the
circuit. Keep in mind that the symbol must accurately indicate the circuit's function for the type of logic
(negative or positive) in use.
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