Digital Communication

Digital Circuits

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  • Question 1

    Don’t just sit there! Build something!!

    Learning to analyze digital circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

    You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

    1. Draw the schematic diagram for the digital circuit to be analyzed.
    2. Carefully build this circuit on a breadboard or other convenient medium.
    3. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
    4. Analyze the circuit, determining all output logic states for given input conditions.
    5. Carefully measure those logic states, to verify the accuracy of your analysis.
    6. If there are any errors, carefully check your circuit’s construction against the diagram, then carefully re-analyze the circuit and re-measure.

    Always be sure that the power supply voltage levels are within specification for the logic circuits you plan to use. If TTL, the power supply must be a 5-volt regulated supply, adjusted to a value as close to 5.0 volts DC as possible.

    One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

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  • Question 2

    Explain the difference between serial digital data and parallel digital data.

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  • Question 3

    The following schematic diagram shows two four-bit universal shift registers used to communicate data serially over a coaxial cable of unspecified length:

    Specify what logic states would have to be input at the PL, CE, and Clk terminals of each shift register, and at what times, to successfully load four bits of parallel data, shift them serially over the coaxial data cable, and then hold them at the outputs (Q) of the receiving shift register.

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  • Question 4

    Personal computers and peripheral devices provide a rich source of examples for both serial and parallel data transmission. Identify some common examples of both serial and parallel data transmission networks (and standards) at work in a common personal computer. Examples may include communication between computers, between computers and peripheral devices (printers, scanners, cameras, special cards), or between fundamental components of the computer (CPU, disk drive, monitor, etc.).

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  • Question 5

    A ubiquitous example of serial data communication is the cable linking a keyboard to a personal computer: for every key switch pressed, an ASCII character is transmitted to the computer. An interesting characteristic of this particular communication protocol is the random rate at which the ASCII characters are sent. Because the characters are generated at the rate the computer user happens to type, the rate is completely unpredictable. Consequently, this form of serial data communication is known as asynchronous.

    Compare and contrast this against synchronous serial data communication, giving an example of a synchronous data communications standard.

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  • Question 6

    An important integrated circuit (IC) used in digital data communication is a UART. Describe what this acronym stands for, and explain the purpose of this circuit.

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  • Question 7

    Shown here are three different telegraph circuits. Determine which of these could be classified as simplex, full-duplex, and half-duplex, in terms of serial data transmission:

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  • Question 8

    An early form of digital, serial data communication was Morse code. Explain what “Morse code” is (or was), and how it compares to more modern codes such as ASCII.

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  • Question 9

    An important performance parameter of digital communications networks is the number of bits per second (bps) of data it can handle. Unfortunately, a different term called baud is often used interchangeably with bps. Define what “baud” is, and how it differs from “bits per second.”

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  • Question 10

    In Claude Shannon’s famous 1948 paper entitled A Mathematical Theory of Communication, he opens with the following statement:

    “The recent development of various methods of modulation such as PCM and PPM which exchange bandwidth for signal-to-noise ratio has intensified the interest in a general theory of communication.”

    Explain what Shannon was referring to when he said, “exchange bandwidth for signal-to-noise ratio”. In many cases, the superior signal-to-noise ratio of digital communication over analog communication is the primary reason justifying the much greater complexity of digital communications equipment. Also, elaborate on how bandwidth becomes sacrificed in order to achieve relatively noiseless signal transmission.

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