Making an 8 bit OR gate with the 74HC541

Discussion in 'General Electronics Chat' started by m4ch1n314ngu4g3, Sep 3, 2013.

  1. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    I am currently nearing the completion of my 8-bit TTL computer and have incorporated the use of buffers in the 74HC541 for isolating signals in the control unit. For example, a single output enable can be receiving inputs from multiple chips, yet each chip also wants to enact another function yet doesn't want it to be enabled by the outputs of the other chips.

    Hence I attached the outputs of the devices connected to the output enable of this device to the inputs of the 74HC541 octal buffer and connected all the outputs together. Hence I believe this should act as an 8 input OR gate where the inputs are isolated and unaffected by each other's state. The 74HC541 has it's enabled pins set to a constant LOW meaning that it's buffers will always be enabled to output.

    But one thing I just wanted to clear up was the idea of how sourcing and sinking works. I know that with a transistor or a buffer, if you set the collect to logical low and connect the emitter to the cathode or an LED whose anode is connected to 3.3V, the LED will be able to light up since the output of the transistor/buffer acts like a ground or 0V terminal for the LED, allowing it to complete the circuit. And it is understood that this 3.3V signal will not be able to make it's way to the collector of the buffer since the buffer is acting like a diode.

    But if I have a group of buffers whose inputs are set to logical low and their outputs connected together, if one of the inputs were set to logical high, which signal would be pulling the other. In general, would the buffer with the 5V input pull up the outputs of all the other buffers whose output is logical low, and one what condition would one buffer outputting logical low pull down the outputs of all the other buffers if they were at logical high?

    So that's that and thank you for your help.
     
  2. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    Big no no.

    You cannot tie the outputs together. They will essentially short each other will possible damaging effects.

    You can do this with open-collector (pull-down) outputs, but not totem pole (push/pull) outputs.
     
  3. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Yeah, that's what I feared. But it worked with my other circuit except that it was a bunch of tri-state buffers with their outputs connected together; yet only one of them was enabled at any time. So what chip would I use that is similar to the 74HC541 that provides the open-collector outputs?

    Thanks
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    If you want a signal that is HI whenever any of eight input signals is HI, then use an OR gate. Push comes to shove, you can build it up out of two quad OR gates (74HC32), though you would have three levels of gate delay.
     
  5. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Yeah, that was what I was thinking, but I felt that the buffers would be faster and easier to expand into a larger gate. I was going to do something similar to a normal transistor OR gate using the 74HC126 tri-state buffers as my transistors which would be faster with one gate delay but taking up more space on the board.

    Anyways, thanks.
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    What's wrong with 8 diodes and one resistor?
     
  7. m4ch1n314ngu4g3

    Thread Starter New Member

    Aug 20, 2013
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    Well, I know that I could use diodes, but I don't have any diodes at the moment but anyways I've got a whole bunch of 74HC126s from a project where I didn't get to use them. Plus there's the whole idea of a fully CMOS or TTL computer. Oh well.
     
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