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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 concurrent section of the hardware description is where the ________ are interconnected.

    • Options
    • A. functions
    • B. components
    • C. circuits
    • D. macrofunctions
      • Discuss
    • 2. Output 6 of a 74138 octal decoder is selected when it is enabled by a data input of:

      A0 = 1
      A1 = 1
      A2 = 0


    • Options
    • A. True
    • B. False
      • Discuss
    • 3. Assume a ROM to be tested is compared with a known good ROM. If the checksums differ, the ROM is ________.

    • Options
    • A. very likely to be good
    • B. definitely good
    • C. very likely to be bad
    • D. definitely bad
      • Discuss
    • 4. The most common TTL series ICs are:

    • Options
    • A. E-MOSFET
    • B. 7400
    • C. quad
    • D. AC00
      • Discuss
    • 5. Microcontrollers often have:

    • Options
    • A. CPUs
    • B. RAM
    • C. ROM
    • D. all of the above
      • Discuss
    • 6. Before an SOP implementation, the expression would require a total of how many gates?

    • Options
    • A. 1
    • B. 2
    • C. 4
    • D. 5
      • Discuss
    • 7. The outputs of a 7485 four-bit magnitude comparator when the inputs are A = 0110 and B = 1010 are:

      A < B is 1
      A = B is 0
      A > B is 1


    • Options
    • A. True
    • B. False
      • Discuss
    • 8. How many outputs are on a BCD decoder?

    • Options
    • A. 4
    • B. 16
    • C. 8
    • D. 10
      • Discuss
    • 9. The GAL16V8 has eight dedicated input pins.

    • Options
    • A. True
    • B. False
      • Discuss
    • 10. The Boolean expression AB + CD is an example of ________.

    • Options
    • A. PAL
    • B. GAL
    • C. SOP
    • D. POS
      • Discuss

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