Strange behaviour when simulating 8 Bit Counter using 74191 devices in Multisim 11

Discussion in 'Homework Help' started by ARRozental, Mar 25, 2015.

  1. ARRozental

    Thread Starter New Member

    Jul 17, 2012
    4
    0
    Hello
    I am attempting to simulate the circuit attached below in Multisim 11. Basically, I am trying to make an 8 bit up/down counter from two 4 bit 74191 up/down counter devies. I have connected the first counter so that it counts up to 15 then halts at 15. The second counter is connected to the first so that when the first counter reaches 15, the second counter begins its count.
    When simulating the circuit, the first counter seems to work fine- counts up to fifteen and halts there. However, I am never given an opportunity to see if the second counter starts counting as the simulation seems to slow to a crawl or stop at that point, and I dont know why this is happening.
    Could someone please take a look at my circuit, or suggest a reason why this is happening?
     
  2. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello ARRozental
    Curiously the same effect in ISIS Proteus.
    There are something’s to improve in your design.
    You have a mix of TTL And CMOS IC's.
    The U3A (2 inputs NAND gate) is connected as inverter.
    Do not let as separate devices. Thus it would be more understandable.
    What about If you remove this U3A (2 Input NAND gate) and instead, add an inverter across the output of U4A (4 input NAND gate) and the input CTEN(4) on IC1 ??
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
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    625
    Can you post a schematic? I don't use simulators... Why is the first counter stopping? It's ripple clock output is the clock for the second counter and it needs to continue counting.

    BR
    Dennis
     
  4. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello dl324

    Sure I can post a schematic. they are below.
    Although the ARRozental's a bit hard to see because of the size.



    ARRozental.png MrCarlos.png
     
    absf likes this.
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    625
    Hi MrCarlos,

    Thanks for posting the schematic. Did you change anything? In your reply, you mentioned something about mixing TTL and CMOS...

    Hi ARRozental,

    Since this is homework, why do you think the circuit is behaving the way it is?

    Can you list the requirements for your solution?

    Are you required to use parallel clocking?

    BR
    Dennis
     
  6. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello dl324

    Yes, I Did some changes, In my design compared with the one of ARRozental.
    As you can see: ARRozenta use 74191N and 4014BP. and I use 74LS191 and 74LS00.
     
  7. dl324

    Distinguished Member

    Mar 30, 2015
    3,248
    625
    Hi MrCarlos,
    Guess we should wait for the OP to respond if s/he hasn't already figured it out.

    BR
    Dennis
     
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