Experiment 3: Logic Gates and Pull-Up and Pull-Down Resistors OJECTIVES After completing this experiment, you will be able to Experimentally verify the truth tables for the NAND and NOR, and inverter gates. Use the NAND and NOR gates to formulate other basic logic gates. MATERIALS NEEDED 7400 quad 2-input NAND gate 7402 quad 2-input NOR gate 7404 NOT gate (inverter) DMM probes THEORY LOGIC GATES Logic gates are the basic building blocks of digital electronic circuits. They are devices that perform a specific logic operation on one or more input signals and produce a single output signal. The most basic logic gates are the NOT gate, AND gate, OR gate, and XOR (exclusive OR) gate. These gates can be combined to create more complex circuits that perform more advanced logic operations. UNIVERSAL LOGIC GATES There are three types of logic gates that are considered to be "universal" because they can be combined to create any other logic gate or digital circuit. These universal gates are: NAND (NOT-AND) gate: This gate performs the opposite function of an AND gate, meaning it produces a low output (0) if all of its inputs are high, and a high output (1) otherwise. NOR (NOT-OR) gate: This gate performs the opposite function of an OR gate, meaning it produces a high output (1) if all of its inputs are low, and a low output (0) otherwise. NOT gate (inverter): As the name suggests, it inverts the input signal, so that a high input (1) produces a low output (0), and vice versa. Since NAND and NOR gates are universal gates, any logic circuit can be implemented using only NAND gates, or only NOR gates. TTL (TRANSISTOR-TRANSISTOR LOGIC) TTL (Transistor-Transistor Logic) is a type of digital logic circuit that uses transistors to switch between the two logic levels of 0 and 1. High = 1 Indeterminate Region Low=0 SV 3V 2.4V 2V IV 0.4V OV Figure 3. 1: TTL Switching Voltages 35 As shown in Figure 3.1, the logic HIGH or binary '1' level is typically represented by a voltage between 2.4V-5V, while the logic LOW or binary '0' level is represented by a voltage between OV-0.4V. The exact voltage levels may vary depending on the specific type of TTL circuit. HOW TO CREATE INPUT SIGNALS IN THE LAB Pull-down and pull-up resistors are used in electronic circuits to establish a known or defined voltage level when a switch is open. They are typically used in digital circuits to prevent floating or undefined states that could lead to unreliable or incorrect readings. +5.0V Table 3. 1 -OVout 1k, Pull-Down Switch State Open Closed Figure 3. 2 PULL-DOWN RESISTOR A pull-down resistor is connected between the signal line and ground. When the switch is open, the pull-down resistor ensures that the voltage is "pulled down" to a LOW level (e.g., 0 volts or ground). This establishes a clear "off" or "0" state for the signal. PULL-UP RESISTOR On the other hand, a pull-up resistor is connected between the signal line and a positive voltage source (e.g., Vcc or +5 volts). When the switch is open, the pull-up resistor "pulls" the voltage up to a high level (e.g., 5 volts). This establishes a clear "on" or "1" state for the signal. A When the switch is closed, the pull-down or pull-up resistor has little effect as the switch takes precedence and overrides the resistor's influence on the signal voltage level. +5.0V Pull-Down OV, '0' 5V, '1' 1k, Pull-Up A -OVout Vin Figure 3. 3 In digital circuits, it's important to have a well-defined voltage level to ensure reliable signal interpretation by the receiving circuitry (such as microcontrollers or logic gates). NAMING PORTS ON A SCHEMATIC To simplify the schematic, we can replace the entire pull-up or pull-down network with just the port. +5.0V Pull-Up 5V, '1' OV, ‘0’ 1k -OVin 36 PRELIMINARY PROCEDURE 1. Read the lab. 2. Number the pins on the gates of each circuit in the procedure. 3. Determine the Prelab X output (1 or 0) column corresponding to each figure in the procedure. PROCEDURE Build Figure 3.2 on a breadboard and use either a switch or a wire to implement the switch. Measure the voltage at node Vout when the switch is open and closed. 1. Switch State Open Close Table 3.2 A B Figure 3. 4 A 0 0 1 1 Table 3. 3 A 2. Build the following circuits and complete their corresponding table. Use a DMM to measure and record the output voltage for each input combination, as well as determine its binary representation. B Inputs Figure 3.5 B 0 1 0 1 Pull-Down, Voltage, Vin Inputs A 0 0 1 S.22 V B 0 1 0 a. Figure 3.4 through 3.13 and Table 3.3 through 3.12, respectively. D 7400 Prelab, X Binary Value, (1 or 0) 7402 Ö 1 Prelab, X Output X 1 Pull-Up, Voltage, Vin 5.02V 0.27mV Output X O Binary Value, (1 or 0) Measured Output Voltage 4.42V 4.34 V 4.84 v 132 V Measured Output Voltage 3.67 v 0.062 V 0.062 V 37 Table 3. 4 A Figure 3.6 Inputs A 1 0 Table 3.5 A Figure 3. 7 Inputs A 1 1 0 Table 3. 6 A Figure 3.8 Inputs A 1 0 1 7404 Prelab Output X D 7400 Prelab Output X ) 7402 Prelab Output X O Output X Output X 1 Output X Measured Output Voltage 0.108 V 4.42V Measured Output Voltage 0.001 V 5.02 V Measured Output Voltage 002 V 0.0654 3.42V 38 Table 3. 7 A ure 3.9 Inputs A 1 0 Table 3.8 B Figure 3. 10 Inputs A A 0 0 1 1 Table 3. 9 Figure 3. 11 B 0 1 0 1 Inputs A 0 0 1 1 B 0 1 0 1 Da 7402 Prelab Output X 7402 7402 Prelab, X 7400 Prelab, X a 7402 Output X 7402 1 1 DA 7400 X Output X Output X Measured Output Voltage 4.17 V 4.16 V Measured Output Voltage ५.2 v 4.2V 4.21 4.2 V Measured Output Voltage 39