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Interesting question

  1. Feb 5, 2007 #1
    I don't like electronics at all.However,it's an interesting question.For OR operation standard circuit,there are two diodes which are forward biased in 1-1 state.I cannot draw the circuit here;but anyone familiar to the circuit undearstands that even if the two diodes were not there, the OR operation could be performed.Then what is the function of those diodes?
  2. jcsd
  3. Feb 5, 2007 #2


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    The diodes have to be there otherwise the inputs are not isolated from each other. This may be fine when using toggle switches, but suppose you have two toggle switches and each one has an LED indicator on it's output. The outputs of the switches are then wired together in OR fashion without diodes. Whenever EITHER switch is thrown both LEDs will light up.
  4. Feb 5, 2007 #3
    'Nova's got it exactly right. Sometimes these so-called "wired or" connections are fine, but it is generally bad practice

    The diodes are also there to protect your electronics. For example, in TTL you represent a logic "0" with a ground connection and a logic "1" with a +5V connection. What would happen if you had a 1-0 connection? You've just shorted your +5V rail to ground! Hope you built in some over current protection!
  5. Feb 5, 2007 #4


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    That's Averagesupernova to YOU buster!
    I have to pick on the newbies a bit. :wink: Welcome to PF!
  6. Feb 5, 2007 #5


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    The TTL wired OR used open collector ICs with an external resistor for pullup.
    No problem with shorting the +5 rail to ground.

    Some of the old 74xx TTL ICs with active pullup could tolerate the cross state condition.
    While it was bad practice as stated, I've seen it done.

    Max toggle rate is somewhat lower without active pullup.
    Same goes when using diodes.
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