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Mixing 4000 and 7400 Series

Discussion in 'Electronic Basics' started by Michael, Mar 16, 2008.

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  1. Michael

    Michael Guest

    Hi,

    I have a circuit that is made up of 4000 series CMOS ICs but it needs a
    magnitude comparator to work and I only have the 74LS85. Is there anyway I
    can use this so I don't have to order the 4063 CMOS comparator?

    Thanks in advance,

    Michael
     
  2. Guest

    Your 4000 series slows down at 5V and your LS blows up over 5V. Is
    that acceptable?

    Mixing logic families is mostly OK but things like reset/preset are
    often logic low for TTL and high for CMOS along with limited drive
    from CMOS. I'm sure most everyone has done it at some time. I know I
    have.

    GG
     
  3. Michael

    Michael Guest

    Your 4000 series slows down at 5V and your LS blows up over 5V. Is
    that acceptable?

    Mixing logic families is mostly OK but things like reset/preset are
    often logic low for TTL and high for CMOS along with limited drive
    from CMOS. I'm sure most everyone has done it at some time. I know I
    have.

    GG

    Thanks,

    The circuit is spread across multiple PCBs with this CMOS/TTL problem on
    it's own board. So in theory I should (and can) increase the supply voltage
    on the CMOS bits of the circuit and keep the TTL parts still at 5V? That
    should reduce the operating speed difference somewhat correct?

    There's no reset type pins it's just purely data that will be sent between
    the two that makes thing's slightly simpler.

    Cheers,

    Michael
     
  4. whit3rd

    whit3rd Guest

    A 4000 series CMOS gate will drive ONE 74LS series standard
    input adequately, provided (1) the CMOS has 5V power supply
    and (2) the two chips are at the same temperature.
    CMOS sinks inadequate current when it's hot for a 74LS
    input when that LSTTL chip is cold.

    And, an output from a 74LS gate will drive a CMOS gate adequately
    if you add a pullup resistor (for highest performance, 4k ohms to +5V).
     
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