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Simple OR TTL gate

  1. Feb 23, 2008 #1
    Hi there,

    I have a simple question.
    OR gate between 2 output TTL signals can be done with diodes and simply join the signals toguether... ?

    Output TTL signals have a pull up resistor included:

    For one output:
    ---------------
    Output level 1) 5 V / -4 ma
    Output level 0) 0.7 V / -16ma


    What happens if we join 2 wires:

    TTL Output 1 ----|<------|
    TTL Output 2 ----|<------|---------- TTL Input


    1 + 1 = 5 V / -8 ma - okay
    0 + 0 = 0,7 V / -32 ma - ... limit for the input TTL device

    0 + 1 = 5 V / -4mA - the level 0, O.7 V can't provide current

    I know, I can do that with TTL components.

    Best Pierre.
     
  2. jcsd
  3. Feb 23, 2008 #2

    NoTime

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    Science Advisor
    Homework Helper

    It's called wired OR.
    It works better if you use Open Collector (OC) devices.
    The 74XX series had several OC devices.
    It's not a recommended practice for non oc devices and tends to cause problems.
    Don't try this with CMOS devices.
     
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