Digital Fundamentals. Lab 4 EX-OR Circuits & Combinational Circuit Design

Transcription

1 Richland College School of Engineering & Technology Rev. 0 B. Donham Rev. 1 (7/2003) J. Horne Rev. 2 (1/2008) J. Bradbury Digital Fundamentals CETT 1425 Lab 4 EX-OR Circuits & Combinational Circuit Design Name: Date: Objectives: To construct and investigate the operation of exclusive-or (EX-OR) circuit To design a combinational logic circuit from a set of system requirements. To apply troubleshooting techniques to determine the most probable cause(s) of a circuit failure. Suggested Reading Chapter 4, Digital Systems, Principals and Applications; Tocci Equipment and Components 7400 IC Quadruple 2-Input NAND Gates 7404 IC Hex Inverters 7408 IC Quadruple 2-Input AND Gates 7432 IC Quadruple 2-Input Positive-OR Gates 7486 IC Quadruple 2-Input Exclusive-OR Gates 3 toggle switches Breadboard & wires VOM 0-10 Volt power supply Logic probe Circuit Simulator (MultiSIM or an equivalent) Introduction Exclusive OR The Exclusive OR (XOR) logic function occurs frequently in digital systems. The XOR is a 2-input logic circuit, whose output is a logic 1, whenever either input is a 1 but not both. The logic expression for an EX-OR is X A B A B A B. Page 1 of 8

2 Combinational Logic Circuit Design Design of a basic combinational logic circuit can be accomplished with the following procedure: 1. Create a truth table based upon the system requirements or problem statement. 2. Write the sum-of-products expression for the output. 3. Simplify the output expression with Boolean theorems or K-map. 4. Implement the circuit for the simplified expression. Basic Troubleshooting There are a number of faults that can occur in a digital system, such as pins shorted to ground or a supply voltage, pins shorted together, or open circuits. These faults can occur internal to an integrated circuit (IC) or at the board level due to shorted pins, poor solder joints, broken pins, or solder bridges. Troubleshooting is a process of analyzing and diagnosing problems in a circuit that is not functioning properly. This process should be done in a sequence of logical and systematic steps. A technician should document each investigative step, including the conditions that a test was performed, and the measured results. This information can prove critical when the circuit fails intermittently or multiple failure modes exist. Before a failure analysis can be performed, the proper circuit operation must be understood. Keys to remember when troubleshooting include: Understand the operation of the circuit and have expected results before taking measurements. Always verify the obvious (e.g. Power supply voltage, component values, device types, etc.) Perform the analysis in a logical manner. Don t perform random tests. Eliminate variables to isolate the failure(s). Identify the fault before making corrections, such as replacing a component. Some failures can be masked by other problems. When a failure is identified and corrected, it may be necessary to re-verify the circuit operation and determine if the existing failure modes still are present or if new symptoms have arisen. Document every step in the analysis process. Data that may seem insignificant at the time may prove to be important later in the process. Page 2 of 8

3 Procedure: 1. Construct the following circuit using a 7486 IC and toggle switches for the inputs A and B. Using a logic probe or VOM, measure and record the output level for each input state in the following truth table. B A X Design a circuit for a car chime based upon the following system requirements. A car chime or bell will sound if the output of the logic circuit (X) is set to a logic 1. The chime is to be sounded for either of the following conditions: If the headlights are left on when the engine is turned off If the engine is off and the key is in the ignition when the door is opened Use the following input names and nomenclature in the design process: E Engine. 1 if the engine is ON and 0 if the engine is OFF L Lights. 1 if the lights are ON and 0 if the lights are OFF K Key. 1 if the key is in the ignition and 0 if the key is not in the ignition D Door. 1 the door is open and 0 if the door is closed X Output to Chime. 1 is chime is ON and 0 if chime is OFF Page 3 of 8

4 a. Create the truth table for given system requirements. L K D E X b. Write a sum-of-products expression for the output (X) c. Use Boolean theorems or a K-map to simplify the expression. Show all of the steps used in the selected simplification process. Page 4 of 8

5 d. Draw the circuit diagram for the simplified expression in procedure c. 3. Construct the car chime circuit in procedure 2d in MultiSIM or a similar circuit simulator. Use a toggle switch for each input. Measure and verify that the output logic matches the expected values in the truth table in procedure 2a. Print out a copy of the schematic and attach it to the lab report. 4. Evaluate the following circuit for each input state and record the expected output in the truth table. Ask your instructor to check your truth table before proceeding to the next steps. A B C X Page 5 of 8

6 5. Refer to the previous circuit while performing the following troubleshooting cases. Document all of the analysis steps taken to form your conclusion, including any assumptions that were made. a. When troubleshooting the circuit, a technician determines that the output, X, is always a logic 1 regardless of the input state. Could a short between VCC and input A cause this failure mode? Explain why or why not. b. The output X is supposed to be LOW when A=0, B=1, and C=0 but a technician measures a logic 1 for the given input state. Could a short between Z1-4 and ground cause this failure mode? Explain why or why not. c. The output X is supposed to be LOW when A=1, B=1, and C=1 but a technician measures a logic 1 for the given input state. Could an internal short between Z4-2 and Z4-3 cause this failure mode? Explain why or why not. Page 6 of 8

7 d. The output of the circuit is LOW for every input state except when A=0, B=0, and C=1. The technician discovers that the Z5 component was a NOR gate rather than a NAND gate. Could this have caused the failure? Explain why or why not. e. While troubleshooting the circuit, a technician determines that the output X is always a logic 0. Could an open line between Z3-3 and Z5-2 cause this failure mode (Assume TTL logic is being used)? Explain why or why not. Page 7 of 8

8 Review Questions: Answer the following questions after the lab is completed. 1. The inputs of a 2-input OR gate are connected to a door and window sensor of an alarm system. When the door or window is opened, the input lines go HIGH. When the output of the OR goes HIGH, the alarm sounds. A technician decides to replace the OR gate with an XOR component. Explain the problem(s) this will cause to the operation of the alarm. 2. Describe how the following circuit will operate if a short to ground occurs at the point indicated. Show all of the analysis steps and explain how you reached your conclusion. Short to Ground Page 8 of 8