Lab1 - Floyd, Digital Fundamentals, Tenth Edition CHAPTER...

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Unformatted text preview: Floyd, Digital Fundamentals, Tenth Edition CHAPTER 1: INTRODUCTORY CONCEPTS Experiment 1 Laboratory Instrument Familiarization Objectives After completing this experiment, you will be able to E Use a digital multimeter (DMM) to measure a specified dc voltage from the power supply. El Use an oscilloscope to measure circuit voltages and frequencies. 3 Set up the function generator to obtain a transistor- transistor logic (TTL) compatible pulse of a spec- ified frequency. Measure the pulse amplitude and the frequency with an oscilloscope. El Construct a digital oscillator circuit on a labora— tory protoboard and measure various parameters with the oscilloscope. Materials Needed Light-emitting diode (LED) Resistors: one 330 0., one 1.0 M), one 2.7 k9 Capacitors: one 0.1 uF, one 100 uF One 555 timer For Further Investigation: Current tracer (H—P 547A or equivalent) Logic pulser (H—P 546A or equivalent) One 100 Q resistor Summary of Theory Laboratory equipment needed for most electronics work includes a DMM, a power supply, a func- tion generator, and a dual—trace analog or digital oscilloscope. This experiment is an introduction to these instruments and to protoboards that are com— monly used to wire laboratory experiments. Since each laboratory will have instruments from differ— ent manufacturers and different models, you should familiarize yourself with your particular lab station using the manufacturer’s operating instructions or information provided by your instructor. There is a wide variety of instruments used in electronics labs; however, the directions in this experiment are general enough that you should be able to follow them for whatever instruments you are using. The Power Supply All active electronic devices, such as the integrated circuits used in digital electronics, require a stable source of dc voltage to function properly. The power supply provides the proper level of dc voltage. It is very important that the correct voltage be set before connecting it to the ICs on your board or permanent damage can result. The power supply at your bench may have more than one output and normally will have a built—in meter to help you set the voltage. For nearly all of the circuits in this manual, the power supply should be set to +5.0 V. When testing a faulty circuit, one of the first checks is to verify that the supply voltage is correct and that there is no ac component to the power supply output. The Digital Multimeter The DMM is a multipurpose measuring instrument that combines in one instrument the characteristics 1 of a do and ac voltmeter, a dc and ac ammeter, and an ohmmeter. The DMM indicates the measured quantity as a digital number, avoiding the necessity to interpret the scales as was necessary on older instruments. Because the DMM is a multipurpose instru— ment, it is necessary to determine which controls select the desired function. In addition, current measurements (and often high—range voltage meas— urements) usually require a separate set of lead con- nections to the meter. After you have selected the function, you may need to select the appropriate range to make the measurement. Digital multime— ters can be autoranging, meaning that the instrument automatically selects the correct scale and sets the decimal place, or they can be manual ranging, meaning that the user must select the correct scale. The voltmeter function of a DlVflVI can measure either ac or dc volts. For digital work, the dc volts function is always used to verify that the dc supply voltage is correct and to check steady-state logic levels. If you are checking a power supply, you can verify that there is no ac component in the supply voltage by selecting the ac function. With ac voltage selected, the reading of a power supply should be very close to zero. Except for a test like this, the ac voltage function is not used in digital work. The ohmmeter function of a DMM is used only in circuits that are not powered. When measur— ing resistance, the power supply should be discon— nected from the circuit to avoid measuring the resistance of the power supply. An ohmmeter works by inserting a small test voltage into a circuit and measuring the resulting current flow. Consequently, if any voltage is present, the reading will be in error. The meter will show the resistance of all possible paths between the probes. If you want to know the resistance of a single component, you must isolate that component from the remainder of the circuit by disconnecting one end. In addition, body resistance can affect the reading if you are holding the con— FIGURE 1—1 Voltage Definitions for a periodic pulse train. ducting portion of both probes in your fingers. This procedure should be avoided, particularly with high resistances. The Function Generator The function generator is used to produce signals required for testing various kinds of circuits. For digital circuits, a periodic rectangular pulse is the basic signal used for testing logic circuits. It is im— portant that the proper voltage level be set up before connecting the function generator to the circuit or else damage may occur. Function gener— ators normally have controls for adjusting the peak amplitude of a signal and may also have a means of adjusting the 0 volt level. Most function generators have a separate pulse output for use in logic cir— cuits. If you have a TTL compatible output, it will be the one used for the experiments in this manual. A periodic rectangular pulse is a signal that rises from one level to another level, remains at the second level for a time called the pulse width, (tw), and then returns to the original level. Important pa— rameters for these pulses are illustrated in Figure 1—1. For digital testing, it is useful to use a duty cycle that is not near 50% so that an inverted signal can be readily detected on an oscilloscope. In addition to amplitude and dc offset controls, function generators have switches that select the range of the output frequency. A vernier control may be present for fine frequency adjustments. The Oscilloscope The oscilloscope is the most important test instru— ment for testing circuits, and you should become completely familiar with its operation. It is a versa- tile test instrument, letting you “see” a graph of the voltage as a function of time in a circuit and compare waves. Because an oscilloscope allows you to measure various parameters, it is considered to be an instrument capable of parametric measurements important in both digital and analog work. Nearly Leading edge (positive slope) Trailing edge (negative slope) Amplitude Duty cycle = 100% A duty cycle of 25% is shown. all complex digital circuits have specific timing re— quirements that can be readily measured with a two— channel oscilloscope. There are two basic types of oscilloscopes: analog and digital. Because of its versatility, accu- racy, and ability to do automated measurements, digital scopes are the choice of many technicians today. Both types of scopes have four main control groups: display controls, vertical and horizontal controls, and trigger controls. If you are not familiar with these controls, or the operation of the oscillo- scope in general, you should read the Oscilloscope Guide starting on page Both analog and digital scopes are covered in this summary. In addition, you may want to review the operator’s manual that came with the oscilloscope at your lab station. Logic Pulser and Current Tracer The logic pulser and current tracer are simple digital test instruments that are useful for finding certain dif- ficult faults, such as a short between VCC and ground. A problem like this can be very difficult to find in a large circuit because the short could be located in many possible places. The current tracer responds to pulsing current by detecting the changing magnetic field. A handheld logic pulser can provide very short duration, nondestructive pulses into the shorted circuit. The current tracer, used in conjunction with the pulser or other pulsating source, allows you to follow the current path, leading you directly to the short. This method of troubleshooting is also useful for “stuc ” nodes in a circuit (points that have more than one path for current). The sensitivity of the current tracer can be varied over a large range to allow you to trace various types of faults. Logic Probe Another handheld instrument that is useful for tracing simple logic circuits is the logic probe. The logic probe can be used to determine the logic level FIGURE 1—2 Tektronix logic analyzers (courtesy of Tektronix, Inc.). of a point in a circuit or to determine whether there is pulse activity at the point by LED (light—emitting diode) displays. Although it is used primarily for simple circuits because it cannot show important time relationships between digital signals, a good probe can indicate activity on the line, even if it is short pulses. A simple logic probe can determine if logic levels are HIGH, LOW, or INVALID. Logic Analyzer One of the most powerful and widely used instru— ments for digital troubleshooting is the logic ana- lyzer. The logic analyzer is an instrument that origi- nally was designed to make functional (as opposed to parametric) measurements in logic circuits. It is useful for observing the time relationship between a number of digital signals at the same time, allowing the technician to see a variety of errors, including “stuck” nodes, very short noise spikes, intermittent problems, and timing errors. Newer analyzers can include multiple channels of a digital storage oscillo- scope (DSO) as well as logic channels. An example of a two-function analyzer that can be equipped with multiple channels of D80 and as many as 680 logic analyzer channels is the Tektronix TLA700 series shown in Figure 1—2. Not all electronic laboratories are equipped with a logic analyzer, even a simple one, and one is not necessary for the experiments in this manual. Further information on logic analyzers is given on various websites on the Internet (see www.Tektronix.corn for example). Protoboards Protoboards are a convenient way to construct cir- cuits for testing and experimenting. While there are some variations in the arrangement of the hole pat- terns, most protoboards are similar to the one shown in Figure 1—3, which is modeled after the Radio Shack board 276- 174. Notice that the top and bottom horizontal rows are connected as a continuous row. This horizontal row is connected together. Each group of 5 holes in a vertical row is connected together. Integrated circuits are inserted across center divider. Pin 1 FIGURE 1—3 Power or ground bus ,"E- Power or ground bus Protoboard. An 8—pin integrated circuit is shown inserted into the board. Vertical groups of five holes are connected together; the vertical group above the center strip is not con- nected to the vertical group below the center strip. The holes are 0.1 inch apart, which is the same spacing as the pins on an integrated circuit DIP (dual in~line pins). Integrated circuits (ICs) are in- serted to straddle the center; in this manner, wires can be connected to the pins of the IC by connecting them to the same vertical group as the desired pin. Pin Numbering Integrated circuits come in various “packages” as explained in the text. In this manual, you will be using all “DIP chips”. To determine the pin numbers, you need to locate pin 1 by looking for a notch or dot FIGURE 1—4 Prototyping system (courtesy of National Instruments). 1. Laptop Computer 2. USB Cable 3. NI USB M Series with Mass Termi- nation Device 4. NI USB M Series Device Power Cord Shielded Cable to M Series Device 6. NI ELVIS Benchtop Workstation 5" on one end (see Figure 1—3). Pin 1 is adjacent to this notch as shown. The numbering for a DIP chip always is counterclockwise from pin 1. Prototyping System In many engineering and educational laboratories, the instruments described previously are combined into a data acquisition system that can collect and measure signals and show the results on a computer display. Systems like this are a complete prototyp- ing system integrated into a workstation. An ex- ample is the National Instruments ELVIS system and data acquisition device, shown in Figure 1—4. The workstation has all of the instruments built in and a modular protoboard mounted on top. Procedure Measurement of DC Voltage with the DMM 1. Review the operator’s manual or informa— tion supplied by your inst1uctor for the power supply at your lab station. Generally, power supplies have a meter or meters that enable you to set the output voltage and monitor the current. Set the voltage based on the power supply meter to +5.0 V and record the reading in Table 1—1 (in the Report section). 2. The +5.0 V is the voltage you will use for nearly all of the experiments in this manual. For most TTL circuits, the power supply should be from 4.75 V to 5.25 V. To check that you have correctly set up the supply, measure the voltage with the DMM. Record the reading of the DMM in Table 1—1. Measurement of DC Voltage with the Oscilloscope 3. In this step, you will confirm the dc voltage from the power supply using the oscilloscope. Set the SEC/DIV control of your oscilloscope to a con- venient value (a value near 0.2 ms/div is suggested to give a steady line on the display). Set the trigger controls to AUTO and INT (internal trigger) to assure a sweep is on the display. Select channel 1 as the input channel, and connect a scope probe to the vertical input. Put the input coupling control on GND to disconnect the input signal and find the ground position on the oscilloscope (digital scopes may have a marker for the GND level, as illustrated in item 13 of Figure I—4). Adjust the beam for a sharp, horizontal line across the scope face. 4. Since you will be measuring a positive voltage, position the ground on a convenient grati- cule line near the bottom of the display using the vertical POSITION control. If you are using an analog scope, check that the vertical VOLTS/DIV variable knobs are in their calibrated positions. A digital scope, such as the Tektronix TD82024, is always calibrated, and there is no vernier control. 5. Move the channel 1 input coupling control from the GND position to the dc position. For almost all digital work, the input coupling control should be in the DC position. Clip the ground lead of the scope probe to the ground of the power supply and touch the probe itself to the power supply output. The line on the face of the oscillo- scope should jump up 5 divisions. You can deter— mine the dc voltage by multiplying the vertical sensitivity (1.0 V/div) by the number of divisions observed between ground and this line (5 divisions). Record the measured voltage (to the nearest 0.1 V) in Table 1—1. Measurement of Pulses with the Oscilloscope 6. Now you will set up the function generator or pulse generator for a logic pulse and measure some characteristics of the pulse using the oscillo— scope. Review the operator’s manual or information supplied by your instructor for the function genera- tor at your lab station. Select the pulse function and set the frequency for 1.0 kHz. (If you do not have a pulse function, a square wave may be substituted.) 7. Set up and measure the pulse amplitude of the function generator. The vertical sensitivity (VOLTS/DIV) control of the oscilloscope should be set for 1.0 V/div and the SEC/DIV should be left at 0.2 ms/div. Check that both controls are in their cal- ibrated positions. Check the ground level on the oscilloscope as you did in Step 3 and set it for a con- venient graticule near the bottom of the scope face. Switch the scope back to dc coupling and clip the ground lead of the scope probe to a ground on the generator. Touch the probe to the function genera- tor’s pulse output. If the generator has a variable amplitude control, adjust it for a 4.0 V pulse (4 divi— sions of deflection). Some generators have a sepa— rate control to adjust the dc level of the pulse; others do not. If your generator has a dc offset control, adjust the ground level of the pulse for zero volts. 8. You should obtain a stable display that allows you to measure both the time information and the voltage parameters of the waveform. (If the wave— form is not stable, check triggering controls.) In Plot 1 of your report, sketch the observed waveform on the scope display. It is a good idea, whenever you sketch a waveform from a scope, to record the VOLTS/DIV and SEC/DIV settings of controls next to the sketch and to show the ground level. Measure the pulse width (tw), period (T), and amplitude of the waveform and record these values in Table 1—2. The amplitude is defined in Figure 1—1 and is measured in volts. 9. Connect the LED and series—limiting resis— tor, Rl, to the pulse generator as shown in Figure 1—5. Note that the LED is a polarized component and must be connected in the correct direction to work. The schematic and an example of protoboard wiring are shown. Measure the signal across the LED with the oscilloscope and show it in Plot 2 of your report. Label the scope settings as in step 8 and show the ground level. 10. Sometimes it is useful to use an oscillo- scope to measure the voltage across an un- grounded component. The current—limiting resistor, R1, in Figure 1—5 is an ungrounded component. To measure the voltage across it, connect both channels of your oscilloscope as shown in Figure 1—6. Make sure that both channels are calibrated and that the 5 Fun Current limiting Common 0 PUISB t resistor genera or D 1 kHz 2;, LE 0—4 V LED Cathode side /\ Cathode side (Has longer internal lead and flat side) (a) Schematic FIGURE 1—5 Circuit for Step 9. vertical sensitivity (VOLTS/DIV) is 1 V/div for each channel. If you are using a newer scope, the differ- ence operation (Channel 1—Channel 2) is likely to be shown as a menu item. On older scopes, the dif- ference measurement is done by inverting channel 2 and selecting the ADD function. Consult the opera— tor’s manual if you are not sure. Measure the signal across R1 and show the result on Plot 3. As a check, the sum of the voltages across the LED and resistor should be equal to the voltage of the generator. Constructing and Measuring Parameters in a Digital Circuit 11. In this step, you will construct a small digital oscillator. This oscillator generates pulses ction generator Pulse output 0- LED Anode side (b) Protoboard wiring that could be used to drive other digital circuits. The basic integrated circuit for the oscillator is the 555 timer, which will be covered in detail later. The schematic and sample protoboard wiring is shown in Figure 1—7. Construct the circuit as shown. 12. Using your oscilloseope, observe the signal on pin 3. Sketch the observed signal in Plot 4. Be sure to label the plot with the scope settings (VOLTS/DIV and SEC/DIV). Measure the parameters listed in the first four rows of Table 1—3. The frequency is com— puted from the period measurement (f = l/T). 13. Replace C1 with a 100 [AF capacitor. The light should blink at a relatively slow rate. A slow frequency like this is useful for visual tests of a circuit or for simulating the opening and closing of a manual switch. Measure the period and frequency FIGURE 1—6 Measuring an un— grounded compo- nent. Both channels must be calibrated and have the same vertical sensitivity settings. On the TD82024, the dif— ference between the two channels is on the MATH func— tions menu. i r i i " r t ' COUPUNG counme i i —u— POSmON i POSJTION : POSll‘rON : 3 s g «o ‘ vouS/DN Voust SEC/DIV t sv 2m sv 2m: 5; SI: SOURCE cm cs 2 E)“ LIE “:7ch mucosa mumu mmmm mmmmm mmmmnl (a) Schematic FIG U RE 1—7 Digital oscillator. of the oscillator with the 100 uF capacitor. This signal, with a low frequency like this, may give you difficulty if you are using an analog scope. You will need to use NORMAL triggering instead of auto triggering and you may need to adjust the trigger LEVEL control to obtain a stable display. Record your measured values in Table 1—3. For Further Investigation Using the Current Tracer If you have a current tracer available, you can test the paths for current in a circuit such as the one you constructed in step 9. The current tracer can detect the path of pulsing current, which you can follow. Function generator Pulse output 0 Power supply +5.0 V Common Current tracer (a) Connection of current tracer FIGURE 1—8 Ground +5.0 V LED (b) Protoboard wiring The current tracer detects fast current pulses by sensing the changing magnetic field associated with them. It cannot detect dc. Set the generator to a 1.0 kHz TTL level pulse for this test. Power the current tracer using a +5.0 V power supply. You will need to provide a common ground for the pulse generator and the power supply, as shown in Figure 1—8(a). (Note that the current tracer has a red wire in one of the leads, which should be connected to the +5.0 V source, and a black wire in the other lead, which should be connected to the common.) The sensitivity of the tracer is adjusted with a variable control near the tip of the current tracer. The current tracer must be held perpendicu— larly with respect to the conductor in which you are sensing current. In addition, the small mark on the O O ::::: R::::: : '0" "‘* CI. .0..- (b) Adding a current path through R; FIGURE 1—9 Stimulating the circuit with a logic pulser. T 0 current tracer probe tip must be aligned with the current path to obtain maximum sensitivity. Begin by holding the current tracer above R1. Rotate the current tracer so that the tip is aligned with the path of current. Adjust the sensi— tivity to about half—brightness. You should now be able to trace the current path through R1, the LED, and along the protoboard. Practice tracing the path of current. Simulate a low-impedance fault by installing a 100 Q resistor (call this R2 for this experiment) in parallel with the LED, as shown in Figure l—8(b). Test the circuit with the current tracer to determine the path for current. Does most of the current go through R2 or through the LED? Using the Current Tracer and Logic Pulser Circuit boards typically have many connections where a potential short can occur. If a short occurs between the power supply and ground due to a solder splash or other reason, it can be diflicult to find. A logic pulser, used in conjunction with a Fl G U RE 1—1 0 Simulating a short circuit. The logic pulser forces current through the short; this current can be detected with the cu1rent tracer. Power supply +5.0 V Common Logic pulser current tracer, can locate the fault without the need for applying power to the circuit. The logic pulser applies very fast pulses to a circuit under test. A flashing LED in the tip indicates the output mode, which can be set to various pulse streams or to a continuous series of pulses. The pulser can be used in an operating circuit without damaging it because the energy supplied to the circuit is limited. Start with the logic pulser by setting it for con- tinuous pulses. Remove the pulse generator from the test circuit and touch the logic pulser to the test circuit, as shown in Figure 1—9. You can hold the current tracer at a 90-degree angle and against the tip of the logic pulser in order to set the sensitivity of the current tracer. You should now be able to follow the path for current as you did before. Now simulate a direct short fault across the circuit by connecting a wire as shown in Figure 1—10. You may need to adjust the sensitivity of the current tracer. Use the logic pulser and current tracer to follow the path of current. Can you detect current in R1? Describe in your report the current path in the wire and in the protoboard. Simulated fault Power supply +5.0 V Common To current tracer Report for Experiment 1 Name:__________ Date:_____ Class: Objectives: El Use a digital multimeter (DMM) to measure a specified dc voltage from the power supply. D Use an oscilloscope to measure circuit voltages and frequencies. B Set up the function generator to obtain a transistor—transistor logic (TTL) compatible pulse of a specified frequency. Measure the pulse amplitude and the frequency with an oscilloscope. |Z| Construct a digital oscillator circuit on a laboratory protoboard and measure various parameters with the oscilloscope. Data and Observations: TABLE 1—1 Voltage Setting = 5.0 V Voltage Reading Power supply meter DMlVI Oscilloscope T 7 TABLE 1—2 Function Generator Measured Parameters (at 1.0 kHz) Values I IE I. mm“ u u a u Period I I I ' Amplitude PLOT 1 Generator waveform PLOT 2 Voltage across LED T PLOT 4 Digital oscillator output (pin 3) Results and Conclusion: Further Investigation Results: 10 PLOT 3 Voltage across R1 TABLE 1—3 Measured Values Digital Oscillator Parameters Step Period 12 Duty cycle Amplitude Frequency 13 Period Frequency Evaluation and Review Questions 1. Why is it important to check the dc voltage from a power supply before connecting it to a logic circuit? 2. Both analog and digital oscilloscopes have four major categories of controls. In your own words, explain the function of each section: a. vertical section b. trigger section c. horizontal section d. display section 3. Explain how to measure voltage across an ungrounded component with a two—channel oscilloscope. 4. In Step 11, you constructed a digital oscillator. Assume each of the following faults were in the circuit (each fault is independent of the others). Explain what symptom you would expect to observe with an oscilloscope. a. The LED is inserted in reverse. b. The value of C1 is larger than it should be. 11 c. The power and ground connections on the power supply were accidentally reversed. (Don’t test this one!) d. R1 is open. 5. Compare the advantage and disadvantage of making a dc voltage measurement with a DMM and a scope. 6. Explain how a logic pulser and a current tracer can be used to find a short between power and ground on a circuit board. 12 ...
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Lab1 - Floyd, Digital Fundamentals, Tenth Edition CHAPTER...

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