RS flip flop -- Replacing a NOR gate with an OR gate

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Discussion Overview

The discussion revolves around the behavior of RS flip flops when replacing a NOR gate with an OR gate. Participants explore the implications of this change on the outputs and the fundamental requirements for bistable circuits.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant asserts that replacing the NOR gate with an OR gate results in indeterminate outputs, specifically that both Q and Q' will yield 1 when both inputs are 0.
  • Another participant suggests a methodical approach to determine outputs by varying the Set and Reset inputs, starting from both being 1 and then both being 0.
  • A participant adds a diagram link to illustrate the RS flip flop using a NOR gate.
  • One participant mentions a general principle that a bistable circuit requires an inverting function, proposing that XOR gates could potentially be used, though they express uncertainty about this approach.
  • A later reply challenges the feasibility of creating a bistable circuit with just two XOR gates, emphasizing the necessity of an inverting logic function.
  • Another participant agrees that a solution with only XOR gates is not possible and also questions the viability of using diode-only logic for this purpose.

Areas of Agreement / Disagreement

Participants express differing views on the outputs of the RS flip flop when changing the gate type, and there is no consensus on the possibility of creating a bistable circuit with XOR gates or diode-only logic.

Contextual Notes

Participants discuss the implications of changing gate types without resolving the underlying assumptions about the behavior of bistable circuits and the necessity of inverting functions.

Flying_Dutchman
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In the RS flip flops using a NOR gate if we replace the NOR by an OR gate all the outputs will be indeterminate. Both outputs Q and Q' will every time yield 1. Can you explain how the inputs 0 and 0 will give 1 1 as output
 
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Try this,
Start with Set and Reset both being 1, and determine the outputs Q and Q-bar.
Then change either input ( or both ) to 0, and determine the outputs.

Start with Set and reset both being 0, and determine the outputs.
Change either input to a 1 ( or both ), and determine the outputs.

Repeat at your leisure.
 
Adding a diagram to help the OP:

https://commons.wikimedia.org/wiki/File:RS_Flip-flop_(NOR).svg

1586531521990.png
 
Afaik, there's a general principle that a bistable circuit needs some Inverting Function in there. I guess you could also make a bistable with XOR gates but I don't have a handy pencil at the moment.
 
sophiecentaur said:
I guess you could also make a bistable with XOR gates but I don't have a handy pencil at the moment.
Please find a pencil and see if it can be done with two 2-input XOR gates.
 
Dang - I knew I'd be called out on this one!
No chance with just two XOR gates. But any solution has to have an inverting logic function.
I don't think there would be a solution with Diode-only Logic either.
 
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