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Ic 7400

  1. Mar 7, 2008 #1
    What is the maximum no. of outputs, from different 7400 gates that could be connected together ?

    How many gates can be connected to the output of a 7400 gate ?

    The answer to the 2nd question according to me is that if the output of a 7400 gate is high then the no. of gates connected depend on the output voltage , if it goes below 2v then the gate connected will have no input .

    If the output is low , no input can be connected .

    The ans. to 1st question might be that the combination of resultant output must lie b/w 5 & 2v , for high and (0-0.8v ) for low , but i am not sure of a maximum no . ?
  2. jcsd
  3. Mar 7, 2008 #2


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    You should never connect the outputs of gates together; this will cause a short and potentially damage the gates.

    There is no limit to the number of gate inputs you can connect to a single gate output (this is called fanout). You can connect tens of thousands of gates to one output, if you want. The trade-off is that the enormous capacitance of all those gates will make your circuit incredibly slow. Each gate can only supply a certain amount of current, and if you load it with an enormous capacitance, it will take a long time to charge or discharge that capacitance. The general sweet-spot for digital logic, to obtain the best timing, is a fanout of around four. If you're not concerned about speed, then it doesn't matter.

    - Warren
  4. Mar 10, 2008 #3
    You might be able to connect thousands of CMOS 74HC inputs to a 74HC gate output, but that's not true for 7400 TTL.

    Quoting from the book of words produced by Texas Instruments for 7400 series logic: "Each standard output at a low level is capable of sinking current from 10 standard loads of its own series (20 standard loads for series 74ls/74als) and at a high level is capable of supplying current to either 10 or 20 loads of its own series".

    This is known as "fan out".

    Unless the outputs are open collector and thus suitable for "wire or" connection, they should not be connected together or you'll let the smoke out of the package.
  5. Mar 10, 2008 #4


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    That's true, zeitghost. I don't know whether or not the OP is using CMOS or TTL gates, and I just assumed CMOS.

    - Warren
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