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

• Flying_Dutchman
In summary, the conversation discusses the use of NOR and OR gates in RS flip flops and how changing the inputs can affect the outputs. The speaker suggests trying different input combinations and provides a diagram for reference. They also mention the need for an inverting logic function in bistable circuits and contemplate if a bistable can be made with just XOR gates. The speaker later acknowledges that two XOR gates alone cannot create a bistable and doubts that a solution can be found using only diode logic.
Flying_Dutchman
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

Last edited by a moderator:
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.

Adding a diagram to help the OP:

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

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.

Tom.G

## 1. What is an RS flip flop?

An RS flip flop is a type of sequential logic circuit that has two inputs, Set (S) and Reset (R), and two outputs, Q and Q'. It is used to store a single bit of data and has two stable states, either Set (Q=1, Q'=0) or Reset (Q=0, Q'=1).

## 2. How does an RS flip flop work?

An RS flip flop works by using two cross-coupled NOR gates, where the output of one gate is connected to the input of the other gate, and vice versa. The Set (S) and Reset (R) inputs control the state of the flip flop by setting or resetting the outputs accordingly. When both inputs are low, the flip flop remains in its current state, and when both inputs are high, the flip flop enters an unstable state.

## 3. Why would you want to replace a NOR gate with an OR gate in an RS flip flop?

Replacing a NOR gate with an OR gate in an RS flip flop can simplify the circuit and reduce the number of gates required. It can also improve the speed and efficiency of the flip flop by reducing the propagation delay.

## 4. What is the difference between a NOR gate and an OR gate in an RS flip flop?

The main difference between a NOR gate and an OR gate in an RS flip flop is that a NOR gate has an active-low output, while an OR gate has an active-high output. This means that the NOR gate outputs a high signal when both inputs are low, while an OR gate outputs a low signal when both inputs are low.

## 5. Are there any disadvantages to replacing a NOR gate with an OR gate in an RS flip flop?

One potential disadvantage of replacing a NOR gate with an OR gate in an RS flip flop is that it may increase the complexity of the circuit. This is because an OR gate requires an additional inverter to achieve the same functionality as a NOR gate. Additionally, an OR gate may be more susceptible to noise and glitches than a NOR gate, which could affect the reliability of the flip flop.

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