AND, OR, INVERTER and NOR Logic Gates
The AND gate produces a logic 1 (hi) output only when all inputs are 1. For all other input combinations the output is a 0 (lo) logic. The wiring diagram and spec sheet for a 7408, a quad two-input AND gate, as well as the rest of logic gates used in this class are available as links on the course web site.
- Place a 7408 chip on your board. Connect pin 14 to the +5 volts source and pin 7 to ground.
- Connect the inputs of one of the AND gates to the logic switches and the output to an LED. Verify and record the truth table of the gate.
- Using two gates wire a 3-input AND gate by cascading two 2-input AND gates. Verify and record the truth table.
- Define in words the operation of a 3-input AND gate.
The OR gate will produce a zero output (lo) if and only if all inputs are zero (lo). Repeat the steps above (AND Gate) for the 7432 OR gate.
An INVERTER as the name implies inverts the input signal. The 7404 is a hex inverter chip. To verify the operation of an inverter, connect three inverter gates in series with the outputs to LED indicators. Record the outputs of each stage for all possible inputs.
Figure 1 - Cascaded INVERTERS
A NAND gate is an AND gate with an inverter at the output.
- Verify and record the truth table of a 2-input NAND gate.
- Write the truth table for a 3-input NAND gate. Implement the cascaded circuit of Figure 2 that uses two 2-input NAND gates. Observe its truth table and compare it with the truth table for the 3-input NAND gate. Are the two circuits equivalent?
Figure 2- Cascaded NAND Gates
A NOR gate is an OR gate with inverted output. The 7402 is a quad 2-input NOR gate. Repeat the procedures above (NAND Gate), using NOR gates instead.