Chem 8153 April 2009 1 Laboratory 9 Digital Circuits The goals in this laboratory session are: (1) Confirm the response of basic logic gates (2) Define good HIGH and LOW inputs (3) Practice using a decoder Introduction In this experiment, you will investigate the properties of NAND and NOR gates. Components and Equipment Needed 1. 7400 TTL NAND gates 2. 7402 TTL NOR gates 3. Resistors 4. LED’s 5. Multimeter 1. Basic NAND and NOR Gate Logic Use a 7400 (TTL, transistor-transistor logic) two-input NAND gate and Vcc = + 5 V for all parts of this experiment. (a) Verify the truth table for your gate by measuring the actual electrical voltages at A, B, and F for the four possible combinations of the switches S1 and S2. The truth table listing the experimental voltage values instead of the corresponding binary 0s and 1s is called the electrical truth table. (b) Experimentally determine the F1 and F2 output waveforms. (c) Repeat (a) for a 7402 TTL NOR gate. 2. Good TTL Logic Highs and Lows S1S23F12AB+5 V312+5 V312F2F1Chem 8153 April 2009 2 Use a 7400 two-input NAND gate. A good TTL high is >2.0 V, and a good TTL low is <0.8 V. (a) Verify that F = A if the two NAND inputs are tied together, by slowly varying VA from 0 to Vcc and measuring VF with an oscilloscope. Graph VF versus VA. VA can be the output of a variable-voltage power supply, but VA should not exceed Vcc = 5 V. Graph VF versus VA. What is the maximum VA for which VF > 2.4 V? What is the minimum VA for which VF < 0.4 V? (b) High output source current: Measure Vout as a function of R. Plot Vout versus Iout. Try R = 1 MΩ , 10 kΩ, 1 kΩ, 470 Ω, 220 Ω, and 100 Ω. What is the maximum Iout (“source current”) for which Vout > 2.0 V (a good TTL logic high)? (c) Low output sink current: Measure Vout as a function R. Plot Vout versus Is (the sink current) . Try R = 1 MΩ, 10 kΩ, 1 kΩ, 470 Ω, 220 Ω, and 100 Ω. What is the maximum Is the output can sink and keep Vout > 2.0 V (a good TTL logic low)? (d) B = 1. A = ?. Note: If A and B are both good logic highs, then F is low. 3120 V to VccFA+ 5 V312R+-VoutIout312F+-VoutIinRAB+5 V 312R+-VoutIs+5 V+5 VChem 8153 April 2009 3 Measure VA and Vout as function of Iin by varying R. Note that the lower R, the larger Iin and the better the logic high at A. Plot VA versus Iin. What is the minimum Iin for A to be a good logic high, i.e., to keep Vout less than 0.4 V? Try R = 1 MΩ, 100 kΩ, 10 kΩ, 1 kΩ. (e) B = 1. A = ?. Note that if A and B are both good logic lows, then F is high. Measure VA and Vout as function of Isi (the input sink current) by varying R. Note that the smaller R, the better the logic low at A. Plot VA versus Isi. What is the maximum Isi for A to be a good logic low, i.e., to keep Vout more than 2.0 V? Try R = 100 kΩ, 10 kΩ, 1 kΩ, 470Ω, and 100Ω. (f) Floating (unconnected) inputs: Measure Vout and infer what A must be: 0 or 1? What does a floating TTL input act as? Note: In actual circuits never leave an unused input floating. Either ground it or connect it through a 1 kW resistor to Vcc. 3. LEDs and Decoders (a) Observe the LED (light-emitting diode) light output as a function of current by varying R and measuring VR. Approximately how much current I is necessary to make the LED appear bright in a lighted room? Approximately what is the voltage drop across the LED when it is lit? Is the voltage essentially constant as long as the LED is lit? Try R = 330 kΩ. 312F+-VoutIsiRAB+5 V312FVoutAB+5 V123FVoutABRVRI+5 VLEDChem 8153 April 2009 4 (b) For what input is the LED on? Can a single 7400 gate light one LED? Two LEDs? (c) For what input is the LED on? Can a single 7400 gate light one LED? Two LEDs? (d) Construct your own four-bit decoder as a multiple gate circuit: Design a combination of gates that will make F = 1 only for the four-bit input ABCD = 1001. For example, the decoder for ABCD = 1110 would be: Use an LED to indicate when F = 1. You can use NAND (7400), NOR (7402), NOT (7404), AND (7408), OR (7432), and XOR (7486) gates. How many possible input four-bit words ABCD are there? Report. Include graphs and answers to questions in green. Draw the respective circuits for each graph requested and to explain your answers. LED312R = 330 !S+5 VLED312R = 330 !S+5 V+5 V DCBADC D.A B.F = (A B) (C D) = A B C D... .