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Chapter 2

Chapter 2. Digital Electronic Signals and Switches. 1. Objectives. You should be able to: Describe the parameters of digital vs. time waveforms. Convert a periodic waveform between frequency and period. 2. Objectives. You should be able to:

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Chapter 2

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  1. Chapter 2 Digital Electronic Signals and Switches 1

  2. Objectives • You should be able to: • Describe the parameters of digital vs. time waveforms. • Convert a periodic waveform between frequency and period. 2

  3. Objectives • You should be able to: • Sketch the timing waveform for a binary string in parallel and serial forms. • Discuss switch and relay applications • Explain the characteristics of diodes and transistors when forward and reverse biased. 3

  4. Objectives • You should be able to: • Calculate output voltage in circuits containing diodes or transistors used as digital switches. • Perform I/O timing analysis in circuits containing relays or transistors. • Explain the operation of a common-emitter transistor circuit used as a digital inverter. 4

  5. Digital Signals • Timing diagram • Voltage versus time • Shows logic state • If not exactly 0V and 5V • Use an oscilloscope to view 5

  6. Clock Waveform Timing • Periodic clock waveform • Repetitive form • Specific time interval • Successive pulses identical • Period • Frequency • F = 1/tp and tp = 1/f 6

  7. Practice Problem Draw timing diagrams for the following circuits: 7

  8. Engineering Notation See Table 2-1 in your text 8

  9. Discussion Points • What does the vertical scale of an oscilloscope represent? • What does the horizontal scale of an oscilloscope represent? • Describe frequency and period. • What is the period of a 75 MHz waveform? • What is the frequency of a waveform with a period of 20 ns? 9

  10. Serial Representation • Single electrical conductor • Slow • One bit for each clock period • Telephone lines, intra-computer • COM ports • Plug-in cards 10

  11. Serial Representation • Several standards • V.90, ISDN, T1, T2, T3, USB, Ethernet, 10baseT, 100baseT, cable, DSL • COM - 115 kbps • USB – Different speeds, depending on version 11

  12. Parallel Representation • Separate electrical conductor for each bit • Expensive • Very fast • Inside a computer • External Devices • Centronics printer interface (LPT1) • SCSI (Small Computer Systems Interface) 12

  13. Parallel Representation • LPT1 • 8-bit parallel • 115 kBps • SCSI • 16-bit parallel • 160 MBps • Bps - BYTES per second 13

  14. Discussion Points • Describe the difference between parallel and serial transmission. • What advantage does parallel transmission have over serial transmission? • Are there any disadvantages to parallel transmission? • How long will it take to transmit two 8 bit binary strings using both serial and parallel if the clock frequency is 25 MHz? 14

  15. Switches in Electronic Circuits • Make and break a connection • Manual switch vs. electromechanical relay • Semiconductor devices • Diodes • Transistors • Manual Switches - ideal resistances: • ON - 0 ohms • OFF - infinite 15

  16. The Relay as a Switch • Electromechanical relay • Contacts • External voltage to operate • Magnetic coil energizes • NC - normally closed • NO - normally open • Provides isolation • Triggering source • Output 16

  17. The Relay as a Switch • Disadvantages • Relatively high current is required • Slow - milliseconds vs. micro or nanoseconds • Energized relay coil • Replace source with clock oscillator • Timing diagrams • See Figure 2-17 17

  18. Figure 2-17 18

  19. A Diode as a Switch • Semiconductor • Current flow in one direction only • Forward-biased • Anode more positive than cathode • Current flow • Reverse-biased • Anode equal or more negative than cathode • No current flow 19

  20. A Diode as a Switch • Analogous to a water check valve • Not a perfect short • See Figure 2-24 • 0.7 V across its terminals 20

  21. Figure 2-24 21

  22. A Transistor as a Switch • Bipolar transistor • Input signal at one terminal • Two other terminals become short or open • Types • NPN • PNP 22

  23. A Transistor as a Switch • NPN • Positive voltage from base to emitter • Collector-to-emitter junction short • ON • Negative voltage or 0 V from base to emitter • Collector-to-emitter junction open • OFF 23

  24. A Transistor as a Switch • PNP • Negative voltage base to emitter • ON • Positive voltage or 0 V from base to emitter • OFF 24

  25. Discussion Points • Name the three pins (leads) of a transistor. • Describe how to turn an NPN transistor ON. • Describe how to turn a PNP transistor ON. 25

  26. The TTL Integrated Circuit • Transistor-transistor logic • Inverter • Provides the complement (inversion) of an input at the output. • Transistor saturation • Transistor cutoff • TTL Integrated Circuit • Totem-pole output 26

  27. The TTL Integrated Circuit • 7404 • Hex inverter • Six complete logic circuits • Single silicon chip • 14 pins • 7 on a side 27

  28. The TTL Integrated Circuit • DIP - dual-in-line package • NC - not physically or electrically connected • Pin configuration • See Figure 2-39 28

  29. Figure 2-39 29

  30. MultiSIM Simulation of Switching Circuits • Simulation software • Overview of operation • Demonstration • Example circuits 30

  31. The CMOS Integrated Circuit • Complementary Metal Oxide Semiconductor • Low power consumption • Useful in battery-powered devices • Slower switching speed than TTL • Sensitive to electrostatic discharge 31

  32. Surface-Mount Devices • SMD • Reduced size and weight • Lowered cost of manufacturing circuit boards • Soldered directly to metalized footprint • Special desoldering tools and techniques • Chip densities increased • Higher frequencies 32

  33. Surface-Mounted Devices • SO (small outline) • Dual-in-line package • Gull-wing format • Lower-complexity logic • PLCC (plastic leaded chip carrier) • Square with leads on all four sides • J-bend configuration • More complex logic 33

  34. Discussion Points • What are some key characteristics of: • TTL devices • CMOS devices • Surface mount devices • From a technician’s standpoint, is there a problem troubleshooting and repairing SMD based equipment? 34

  35. Summary • The digital level for 1 is commonly represented by a voltage of 5 V in digital systems. A voltage of 0 V is used for the 0 level. • An oscilloscope can be used to observe the rapidly changing voltage-versus-time waveform in digital systems. 35

  36. Summary • The frequency of a clock waveform is equal to the reciprocal of the waveform’s speed • The transmission of binary data in the serial format requires only a single conductor with a ground reference. The parallel format requires several conductors but is much faster than serial. 36

  37. Summary • Electromechanical relays are capable of forming shorts and opens in circuits requiring high current values but not high speed. • Diodes are used in digital circuitry whenever there is a requirement for current to flow in one direction but not the other. 37

  38. Summary • The transistor is the basic building block of the modern digital integrated circuit. It can be switched on or off by applying the appropriate voltage at its base connection. • TTL and CMOS integrated circuits are formed by integrating thousands of transistors in a single package. They are the most popular ICs used in digital circuitry today. 38

  39. Summary • SMD-style ICs are gaining popularity over the through-hole style DIP ICs because of their smaller size and reduced manufacturing costs. 39

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