EXCLUSIVE-OR/NOR Gates Digital Logic Fundamentals Exercise 1: EXCLUSIVE OR/NOR Gate Functions EXERCISE OBJECTIVE When you have completed this exercise, you will be able to demonstrate the operation of an EXCLUSIVE- OR and an EXCLUSIVE-NOR logic gate. You will verify your results by generating truth tables for each function. EXERCISE DISCUSSION Pins 14 and 7 supply power to the IC. The IC has four two-input XOR gates labeled A through D. 84 FACET by Lab-Volt
Digital Logic Fundamentals EXCLUSIVE-OR/NOR Gates With a 74LS136 IC, inputs may be tied to other inputs, an output may be connected to inputs, and outputs can be connected to other outputs. The outputs require pull-up resistors. Pin 6 is the output of which XOR gate in the IC? a. gate A b. gate B c. gate C d. gate D The internal open collector output circuit of an XOR gate is shown in the large circle. The output of an open collector device requires an external collector load or pull-up resistor. FACET by Lab-Volt 85
EXCLUSIVE-OR/NOR Gates Digital Logic Fundamentals Output terminals of the 74LS136 require pull-up resistors because the 74LS136 has an open collector circuit at the output. The gate output is pulled up to V CC by a 10 k resistor. The open collector circuit must have a source of power for the output. The pull-up resistor provides this required power source. PROCEDURE Locate the XOR/XNOR circuit block, and connect the circuit shown. Insert a two-post connector in BLOCK SELECT, and place toggle switches A and B of the INPUT SIGNALS circuit block in the LOW position. NOTE: A high logic level turns on an LED. You can verify the state of a signal, as indicated by a circuit LED, by connecting your multimeter to the appropriate test point. 86 FACET by Lab-Volt
Digital Logic Fundamentals EXCLUSIVE-OR/NOR Gates What are the logic levels at XOR gate inputs A and B? a. both low b. both high Based on the input levels of the XOR gate, what is the output state? a. logic 1 b. logic 0 What are the logic levels at XNOR gate inputs A and B? a. both high b. both low Based on the input levels of the XNOR gate, what is the output state? a. logic 1 b. logic 0 FACET by Lab-Volt 87
EXCLUSIVE-OR/NOR Gates Digital Logic Fundamentals Set INPUT SIGNALS toggle switches A and B to HIGH. What is the XOR output logic state? What is the XNOR output logic state? Set INPUT SIGNALS switch A to HIGH and switch B to LOW. What is the XOR output logic state? What is the XNOR output logic state? 88 FACET by Lab-Volt
Digital Logic Fundamentals EXCLUSIVE-OR/NOR Gates Set INPUT SIGNALS switch A to LOW and switch B to HIGH. What is the XOR output logic state? What is the XNOR output logic state? Set INPUT SIGNALS switches A and B to LOW. What is the XOR output logic state? What is the XNOR output logic state? FACET by Lab-Volt 89
EXCLUSIVE-OR/NOR Gates Digital Logic Fundamentals Based on the truth table, which gate detects input inequality by the output being 1? a. XOR b. XNOR Based on the truth table, which gate detects input equality by the output being logic 1? a. XOR b. XNOR What is the relationship of the XOR output logic state with respect to the XNOR output logic state for the same input conditions? a. They are the same. b. There is no relationship. c. They are complementary. Can either input of the XOR gate be used to lock out the effect on the output by the other input? a. yes b. no 90 FACET by Lab-Volt
Digital Logic Fundamentals EXCLUSIVE-OR/NOR Gates CONCLUSION The output of an XOR circuit is high for input conditions of inequality. The output of an XNOR circuit is high for input conditions of equality. The inputs of an exclusive type IC cannot be locked out because all input logic states affect the output state. The outputs of an XOR and XNOR gate are complementary when the input conditions are the same. REVIEW QUESTIONS 1. Which schematic symbol represents an EXCLUSIVE-OR gate? a. (a) b. (b) c. (c) d. (d) 2. Which schematic symbol represents an EXCLUSIVE-NOR gate? a. (a) b. (b) c. (c) d. (d) 3. When the two inputs to an XNOR gate are equal, the output logic state a. is high. b. is low. c. depends on the previous output state. d. cannot be determined. 4. When the two inputs to an XOR gate are not equal, the output logic state a. is high. b. is low. c. depends on the previous output state. d. cannot be determined. 5. In the circuit shown, a. output D is locked out by the action of the pull-up resistor. b. outputs C and D are complementary. c. outputs C and D are in phase. d. outputs C and D are EXCLUSIVE-OR functions of inputs A and B. FACET by Lab-Volt 91