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7402 vs 74L02 vs 74LS02 for Altair Clone

Discussion in 'Electronic Components' started by logjam, Jan 8, 2006.

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  1. logjam

    logjam Guest

    Can someone explain the main differences between those 3 families of
    logic chips?

    I read "standard TTL inputs are stable floating, will read high" and
    that "you can't substitute modern TTL families for standard and expect
    some old equipment to work"

    A project I am working on right now is duplicating an Altair 4k DRAM
    board. I'm preparing to place an order for the ICs. The manual calls
    for 74L02, 74LS00, and a 7406 (for examples). Did they choose these
    different styles (L vs. LS vs. 'nothing') because of cost or were they
    really necessary?

    The manual (link included below) is GREAT. I wish all manuals could be
    this great. :) Not really...

    For example Rev.0 of the board had a IC A, B, R as 74L04s and Rev2
    changes B to an LS04.
    Rev0 has a 7420 for J and rev2 has an L20.
    Revo has a 7400 for L and K, rev2 has a 74LS for L and a 74L00 for K.

    I suppose this could be due to wiring...

    Do I really need to be this specific when ordering new logic chips?

    I'm wondering if I need to special order these parts or if some of them
    will interchange. Will an LS part work in place of an L, or L in place
    of a 'nothing': LS work in the place of a nothing? Do you have any
    suggestions on how I could find information on these old parts?
    Searching in google only results in old stock results.

    Here are 300dpi scans of the board and a datasheet in case you are

    If you're interested in my Altair duplication project send me an e-mail
    and I'll keep you updated.

    1975 4k DRAM kit:
    Top Assembled -
    Top Bare -
    Bottom Bare -
    Manual -
  2. The 74 series is original TTL, power hungry, not so fast.

    74L is a low power version, not not as fast, but with low power
    consumption and low input drive requirements (low fan in), but also
    low output current capability (low fan out).

    74LS is a faster form of the low power TTL that raises the input
    current a bit, increases the output drive current considerable,
    compared to the L series and also increases the speed to faster than
    the original 74 series. The best performance to supply power ratio of
    the three.

    Without reverse engineering the schematic, I can't say why the
    particular examples were chosen, but I doubt price was the issue.

    Today, you have lots more choices, including the 74HCT cmos family,
    though its inputs cannot be left floating.
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