74191 Not Working: Multisim

Discussion in 'General Electronics Chat' started by sjgallagher2, Jan 3, 2014.

  1. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Here's what I've designed:
    [​IMG]
    It's a bit of an eyesore but don't get intimidated, most of it is pulling down the switches. I actually cleaned up a few parts real quick: I used a resistor on the U/D input, and pulled the RCO and Max/Min pins down to ground as well but the result was the same.

    Here's what happens. When I start the simulation, I load in my number (for example, 1111) and start the counter by sending LOAD high. Then the LEDs all light up. They start by showing 1111, then they show 1110, all is good so far. But then, they show 1111 again, and the 10's digit is flickering
    1101
    1111
    1101
    1111
    etc.
    The flickering never stops. I can't seem to find a reason, though I must be overlooking something. All help is greatly appreciated!
     
  2. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    Also: If I delete all pull down resistors the effect remains the same
     
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Maybe it didn't like those LEDs without current limiting resistors...

    Also, never ground your outputs...unless properly specified(rare).
     
  4. sjgallagher2

    Thread Starter Member

    Feb 6, 2013
    111
    7
    I tried that already, but I'll try again just in case I did it wrong the first time (happens)
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    some ttl counters can be preset by pulling down the outputs. that might be whats happening here, even though the 74191 has preset inputs. even the old 7490 can be preset this way.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,356
    Always bare in mind that a simulator might not reflect accurately what happens in a real device.

    Since 7400 devices have relatively high input currents the pull down resistor should be about 220Ω. It would be better to connect the input switches to GND and use 1kΩ pull up resistors.

    In the case with a permanent LOW, as in R2, you may as well connect the input to GND.

    Do not omit current limiting resistors on LEDs. Use 7404 inverters to drive the LEDs by sourcing the LED at Vcc and use the inverter to sink current (through a series resistor, of course).
     
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