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Home ‣ Digital Electronics ‣ MSI Logic Circuits See What Others Are Saying!
  • Question

  • Refer to the display multiplexer given below. The MSD display is blank, while the LSD seems to be OK. The input and output lines to the 74157 and 7449 are checked with a scope and can be seen changing levels. The A input on the 74139 also changes; however, the 0 output on the 74139 is always LOW and the 1 output is always HIGH. The B and EN inputs on the 74139 are always LOW. What could cause the problem and what should be done to correct it?

    Refer to the display multiplexer given below. The MSD display is blank, while the LSD seems to be OK


  • Options
  • A. The 74139 is defective; replace it.
  • B. The EN pin on the 74139 could have a bad connection; check the connection.
  • C. The 74139 is defective and must be replaced; or the EN pin on the 74139 could have a bad connection, which should be checked.
  • D. The MSD display is bad; replace it.

  • Correct Answer
  • The 74139 is defective and must be replaced; or the EN pin on the 74139 could have a bad connection, which should be checked. 

    • More questions

    • 1. The major blocks of the frequency counter are the counter, ________, decoder/display, and the timing and control unit.

    • Options
    • A. signal prescaler
    • B. control inputs
    • C. signal generator
    • D. display register
      • Discuss
    • 2. Shift register counters use logic functions to reset the registers when the desired count is reached.

    • Options
    • A. True
    • B. False
      • Discuss
    • 3. The software used to drive a microprocessor-based system is called FORTRAN.

    • Options
    • A. True
    • B. False
      • Discuss
    • 4. The time delay called access time, tac, is a measure of the ROM's operating speed.

    • Options
    • A. True
    • B. False
      • Discuss
    • 5. The 8051 has ________ 16-bit counter/timers.

    • Options
    • A. 1
    • B. 2
    • C. 3
    • D. 4
      • Discuss
    • 6. The CPU (Central Processing Unit) is composed of memory and outputs.

    • Options
    • A. True
    • B. False
      • Discuss
    • 7. Using separate serial inputs for shifting left or shifting right is a major difference between the 74194 and other shift registers.

    • Options
    • A. True
    • B. False
      • Discuss
    • 8. The time interval on the leading edge of a pulse between 10% and 90% of the amplitude is the rise time.

    • Options
    • A. True
    • B. False
      • Discuss
    • 9. In the keypad HDL encoder, the ts bit array represents a tristate buffer.

    • Options
    • A. True
    • B. False
      • Discuss
    • 10. ________ is a mature technology consisting of numerous subfamilies that have been developed over many years of use.

    • Options
    • A. TTL
    • B. CMOS
    • C. ECL
    • D. None of the above
      • Discuss

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