ECE 320L - Lab 2 - Constructing a Logic probe PDF

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Constructing a Logic probe...

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Spring 2017

California State University, Northridge Department of Electrical & Computer Engineering

Experiment 2 Constructing a Logic probe Jan. 28,2017

ECE 320 L

1. Introduction: In this experiment 2, we construct a logic probe by using 7404 inverter IC. Also, we use this test probe to test another circuit of an IC. Subsequently, we use DMM (digital multimeter) to measure voltages and compare tham with input logic levels that we had before. When we can measure voltage levels, we can determine a logic HIGH or LOW based on values from DMM. 2. Equipment Used: Type

Model 

Digital Multimeter

U3401A

Serial No.

Calibration Date

NI ELVIS Benchtop Workstation 3. Parts Used: QTY

Component 

1

Hex inverter

2

Light-emitting diode

4

Resistors

2

Signal diodes

1

Potentiometer

Value

Type 7404

Three 330 Ohm, one 2.0 Kilo Ohm

Carbon, +/-5% 1N914

1 Kilo Ohm

4. Software Used: MS word 2010 5. Theory : In digital circuits, we have two voltage levels to indicate HIGH and LOW which represent digital binary bit 1 (ON) and 0 (OFF). In this experiment, we will transistor-transistor logic (TTL) (see Figure 2.1). As we can see that, a HIGH logic level is above 2.0 V and a LOW logic level is under 0.8 V. An integrated circuit, or IC, will be used in this experiment. In IC, it has many logic gates for different purposes. In order to power an IC, we need a power (VCC) and a ground connection (GND) to power up an IC. In this lab, we will use 7404 inverter IC to investigate different logic gates (see Figure 2.2). In order to read pins of an IC, we start to count from a top notch (a circle)

which is pin 1. This 704 inverter is an IC that converter logic level from HIGH to LOW or LOW to HIGH depending on an input logic level. (see Figure 2.3) In this experiment, we will use a simple logic probe to detect a HIGH or a LOW logic level in a circuit (see Figure 2.4). If a probe is not connected to any voltages, both LEDs (indicating HIGH and LOW) are not on because an output is HIGH for both of them. They require a LOW logic level to turn on a LED. If a probe input is connected to a voltage approximately 2.0 V, a HIGH LED will turn on to indicate a high logic input. If a probe is lower than 0.8 V, a LOW LED will turn on to indicate a low logic input.

Figure 2.1: TTL (Transistor-transistor-logic)

Figure 2.2: Integrated Circuit (IC) with Pins

Figure 2.3: 7404 Hex Inverter Diagram

Figure 2.4: A Simple Logic Probe

6. Procedure and Results:

As shown above we connected all the parts on the workstation. We tested the circuit by sending a 5V and observing the LED turning on and off. ● We used the 1k Ohm potentiometer to find our high and low threshold. We connected the probe to the potentiometer and used DMM to measure to voltage. ● In order to test the logic gates first we connected the probe to pin 4 of our IC and observed our LEDs with different voltages. ● Next we connected two inverters in series and connected the probe to pin 6. We observed the logic when the input is low, high and open. ●

● As for the next part we connected two inverters as shown below.

● We connected the probe to pin 4 and connect pin 3 with ground. Then we observed the logic when the input is low, high and open. (part 7 on table 2.2). ● As part 8, we gave an input of +5V and recorded the logic level on pin 4. ● For the next part in our experiment we created a fault, we removed the input of pin 5 and connected pin 3 to ground and tested the logic levels. The we measured the voltage levels of each pin with a DMM.

Data and Observations: Step 3: Logic thresholds: HIGH 2.125  V

Step

LOW 0.850  V

Input is LOW

Output Logic Level Input is OPEN

Input is HIGH

4

One inverter

High

Low

Low

5

Two series inverters

Low

High

High

TABLE 2-2 Step

Input Logic Level (pin 3)

Output Logic Level (pin 4)

Logic Level (pin 5)

Logic Level (pin 6)

7

Vin momentarily on ground.

High

8

Vin momentarily on +5.0 V.

Low

9

Fault condition: open at pin 5.

10

Voltages with 0.19 mV fault (DMM):

Low

High

Invalid

Low

3.13 V

1.16 V

0.14 mV

7.Conclusion In this experiment, we can understand how to read pin numbers of an IC and how an IC works in a circuit. Using a simple test probe, we can determine which input is HIGH or LOW from LED indicators. A HIGH logic level is always above 2.0 V and a LOW logic level is always below 1.8 V. A range between these two values (2.0 V and 1.8 V) had invalid values so that neither LOW LED and HIGH LED will turn on. 7404 hex inverter IC is an IC that can convert a HIGH input to a LOW output or a LOW input to a HIGH output. We hope that we can apply this inverter technique for next experiments.

Evaluation and Review Questions 1. In Step 3, you tested the threshold voltages of the logic probe. What simple change to the circuit of Figure 2-4 would you suggest if you wanted to raise these thresholds a small amount?

According to Ohm's law voltage can be increased if we increase the resistance. So if we replace 330 Ohm resistor with a higher resistor, we’d be able to raise the thresholds. 2. In Step 5, two inverters were connected in series; occasionally this configuration is used in logic circuits. What logical reason can you suggest that two inverters might be connected in series like this? When we connect two inverters in series, it amplifies current (allows more currents) for an output logic. Also, an output and input logics will be the same for two inverters in series. 3. Consider the logic drawing in Figure 2-7.

a) Is this the same or different than the circuit in Figure 2-6(b)?.  It is same as the circuit in Figure 2-6(b) b) If the conductor leading to pin 3 is open, what voltage do you expect to see at pin 3? We’d see any invalid voltage. (Between 2.125 -0.8500) c) c. If the conductor leading to pin 3 is open, what voltage do you expect to see at pin 4? We’d see a low voltage 4. Discuss the advantage and disadvantage of using a logic probe and DMM for logic measurements.  Probe is very efficient when we need to find out if the logic level is high or low. But we cannot find out invalid logic levels or exact voltages from the probe. We have to use a DMM for this purpose. 5. Consider the circuit in Figure 2-8 with five inverters. Assume each inverter requires 10 ns for the input logic to affect the output logic (this is called propagation delay).

a. Describe Vout. b. How long does it take for the input to the first inverter to affect the last inverter? 50 ns c. What is the frequency of the output? (Hint: The logic must change twice in one period.) If it change twice in one period, T= 100ns f=1/T f= 1/(100ns) f= 10 MHz 6. When troubleshooting a TTL logic circuit, what is the likely problem if a steady-state input voltage is invalid? It is likely caused by an open input.

References

Buchla, D., & Floyd, T. L. (n.d.). Experiments in digital fundamentals  (10th ed.). (pg 13-19) Upper Saddle River, NJ: Pearson Prentice Hall....