OR Gate Circuit: What is the Function of Diodes?

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The discussion centers on the function of diodes in an OR gate circuit, specifically in TTL (Transistor-Transistor Logic) applications. It is established that diodes are essential for isolating inputs to prevent unwanted interactions between toggle switches, particularly when using LED indicators. The presence of diodes protects the circuit from potential short circuits that could occur if a logic "1" (+5V) and logic "0" (ground) are connected simultaneously. Additionally, the use of open collector ICs with external pull-up resistors is highlighted as a common practice in TTL wired OR configurations.

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Kolahal Bhattacharya
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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?
 
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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.
 
'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!
 
mdelisio said:
'Nova's got it exactly right...

That's Averagesupernova to YOU buster!
-
I have to pick on the newbies a bit. :wink: Welcome to PF!
 
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.
 
Most likely this can only be answered by an "old timer". I am making measurements on an uA709 op amp (metal can). I would like to calculate the frequency rolloff curves (I can measure them). I assume the compensation is via the miller effect. To do the calculations I would need to know the gain of the transistors and the effective resistance seen at the compensation terminals, not including the values I put there. Anyone know those values?

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