how many and gate pins can I output to?

Discussion in 'The Projects Forum' started by Gadersd, Aug 14, 2013.

  1. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
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    I have finally finished the schematic for my ttl computer. I am using a 74ls and gate to zero the outputs of one register. Since my register is 8 bits, I need to send the zero out signal from my decoder to 8 pins on the and gates. How many pins can a 74ls ic output to without needing a buffer to increase the signal?
     
  2. RichardO

    Well-Known Member

    May 4, 2013
    1,230
    382
    You need to check the specification sheets for both the the and-gate output and the register inputs. The spec's will say how many loads can be driven by the gate and how many loads each register input presents to the gate.
     
  3. tshuck

    Well-Known Member

    Oct 18, 2012
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    You will need to check the fan-out for the device, and keep in mind that this value is referenced against a test where the input devices are the same family as the output device.
     
  4. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
  5. Gadersd

    Thread Starter Member

    Dec 8, 2012
    98
    1
    I looked at the datasheets again and saw that the input current of the and gates are 20uA per pin and that the decoder outputs 8mA. 20/1000 = 0.02 8/0.02 = 400 So I can output my decoder into 400 and gate pins?
     
  6. RamaD

    Active Member

    Dec 4, 2009
    254
    33
    400?

    Check for high level output current Vs high level input current, and similiarly for low level. The lower of the two!
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,990
    3,226
    Not even close. ;) You need to look at the low level (logic 0) input current, which is -0.36mA (minus means the current is coming out of the input). For a low level output maximum sink current of 8mA the maximum fan-out is 8mA / 0.36mA = 22.
     
  8. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,356
    Typical fan-out for TTL gates all of the same family is 20.
     
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