RS Flip Flop

The RS flip-flop is used to store binary information (i.e. 0 or 1). It consists of two inputs, SET and RESET. In RS flip-flop ‘R’ Stands for RESET and ‘S’ stands for SET. The flip-flop keeps its present state even when one or both inputs are deactivated. The flip-flop enters the ‘0’ state when the RESET input is activated, and the ‘1’ state when the SET input is activated.

The block diagram of the RS flip-flop is shown above. Since RS flip-flops are used for storing binary information it is used in simple digital applications like data registers and memory cells that require binary storage. Nevertheless, more sophisticated flip-flop designs, such as the D flip-flop or JK flip-flop, are frequently used over the RS flip-flop for their increased reliability and versatility in complex digital systems due to restrictions in handling specific input conditions.

Table of Content

• Construction and Working of RS Flip Flop
• RS Flip Flop Using NAND Gates
• Truth Table for RS Flip Flop
• Characteristic Table for RS Flip Flop
• Excitation Table for RS Flip Flop
• Difference Between SR and RS Flip Flop
• Applications of RS Flip Flop

Construction and Working of RS flip flop

RS flip can be constructed using basic logic gates such as NAND gates or NOR gates. Below shown a RS flip-flop constructed using a NAND gate same we can construct a RS flip flop using a NOR gate.

Working of RS flip-flop depends on its inputs.

• In initial state output can in any state SET or RESET.
• In SET , S=1 and R=0.
• Conversely in RESET , S=0 and R=1.
• When R=0 and S=0 it hold the current state
• When R=1 and S=1 it falls under Undefined / Forbidden state

RS flip-flop Using NAND gates

The RS flip-flop can be constructed using 4 two input NAND gate which is shown in the above figure.

Truth Table For RS Flip-flop

Sl no.	Clock	S	R	Qn+1
1	0	X	X	Qn
2	1	0	0	No change
3	1	0	1	0
4	1	1	0	1
5	1	1	1	Undefined / Forbidden state

Here, S is the Set input, R is the reset input,Qn+1 is the next state and State tells in which state it enters

Characteristic Table For RS Flip-flop

The characteristic equation tells us about what will be the next state of flip flop in terms of present state.

Sl no.	Qn	S	R	Qn+1
1	0	0	0	0
2	0	0	1	0
3	0	1	0	1
4	0	1	1	X
5	1	0	0	1
6	1	0	1	0
7	1	1	0	1
8	1	1	1	X

Characterstic Equation : Qn+1 = S + Qn . R’

Here, S is the Set input, R is the reset input, Qn is the current state input and Qn+1 is the next state outputs.

Excitation table For RS Flip-flop

Excitation Table basically tells about the excitation which is required by flip flop to go from current state to next state.

Sl no.	Qn	Qn+1	S	R
1	0	0	0	X
2	0	1	1	0
3	1	0	0	1
4	1	1	X	0

Here, Qn is the current state, Qn+1 is the next state outputs and S, R are the set and reset inputs respectively.

Difference between SR and RS Flip-Flop

Both SR and RS flipflop are electronic devices used for storing the binary information (i.e. 0 or Both of them are nearly same with more or less same functionality, but differ in their inputs.

SR (‘S‘ stand for SET and ‘R’ stand for RESET ) flip flop has inputs SET and RESET. The flip-flop’s output changes depending on the inputs, with ‘1’ triggering the SET state, ‘1’ triggering the RESET state, and ‘0’ indicating the current state.

Here SET has high priority when both the inputs are ‘1’.

RS (‘R‘ stand for RESET and ‘S’ stand for SET ) flip flop has inputs RESET and SET. The flip-flop maintains its current state when both inputs are ‘0’, as it enters the RESET state when the R input is set to ‘1’.

Here RESET has high priority when both the inputs are ‘1’.

In conclusion to difference between them we can say that both of them are nearly same with more or less same applications and functionality but differ in their input types.

Applications of RS Flip-flop

RS flipflop are data storage device used to store binary information. It is used is mainly devices requires binary information. Its used in:

1. Asynchronous Counters (RS flipflops are used in building the asynchronous counters ).
2. Shift Registers (RS flip-flops can be used to build shift registers).
3. State Machines.
4. Debouncing Circuit (used in stabilizing output of a switch/button).
5. Address Decoding.
6. Clock Synchronization.

Conclusion

RS flip are used to store binary information (i.e. 0 or 1 ). Therefore it is used in data registers and memory cells that requires memory storage . A single RS flip flop will store a single binary digit. Therefore in order to store more binary digits , more RS flip flops will be required. The flip-flop may switch between the binary numbers 0 and 1 because of its operation being controlled by the Reset and Set inputs. Although its simple design makes it useful for simple binary storage jobs, care must be taken to prevent triggering both inputs at once since this can result in unpredictable behavior. D flip flops and JK flip flops are used over RS flip flip for their for their increased reliability and versatility in complex digital systems depending upon the requirement.

FAQs on RS Flip-Flop

Q.1: In which condition the RS flip flop are unstable?

Answer:

When ‘S’ = 1 and ‘R’ = 1 , it falls under unstable of Forbidden state.

Q.2: Difference between RS latch and RS flip-flop.

Answer:

An RS latch is level-sensitive, responding to continuous input signals, while an RS flip-flop is edge-triggered, responding to transitions at specific clock edges.

Q.3: What are the limitations of RS flip-flop?

Answer:

Limitations of RS flip-flop are :-

• Race condition (when S and R are set to ‘1’ and then input changes to another condition, then output become unpredictable)
• Undefined / Forbidden state (S=1 and R=1)
• Complexity in managing in multi bit storage
• Problems caused in clock Synchronization

Q.4: How many RS flipflops are required to store 10 bit data values ?

Answer:

To store a 10-bit binary data value, we would need 10 RS flip-flops. Since a single RS flip flop will store a single binary digit. In order to store 10 bit data value we need 10 RS flip-flops.