The flip-flop is an important element of such circuits. It has the interesting property of An SRFlip-flop has two inputs: S for setting and R for Resetting the flip- flop : It can be set to a state which is retained until explicitly reset.
A flip-flop, as stated earlier, is a bistable circuit. Both of its output states are stable. The circuit remains in a particular output state indefinitely until something is done to change that output status. Referring to the bistable multivibrator circuit discussed earlier, these two states were those of the output transistor in saturation (representing a LOW output) and in cut-off (representing a HIGH output). If the LOW and HIGH outputs are respectively regarded as ‘0‘ and ‘1‘, then the output can either be a ‘0‘ or a ‘1‘. Since either a ‘0‘ or a ‘1‘ can be held indefinitely until the circuit is appropriately triggered to go to the other state, the circuit is said to have memory. It is capable of storing one binary digit or one bit of digital information. Also, if we recall the functioning of the bistable multivibrator circuit, we find that, when one of the transistors was in saturation, the other was in cut-off. This implies that, if we had taken outputs from the collectors of both transistors, then the two outputs would be complementary.
In the flip-flops of various types that are available in IC form, we will see that all these devices offer complementary outputs usually designated as Q and Q‘ The R-S flip-flop is the most basic of all flip-flops. The letters ‘R‘ and ‘S‘ here stand for RESET and SET. When the flip-flop is SET, its Q output goes to a ‘1‘ state, and when it is RESET it goes to a ‘0‘ state. The Q‘ output is the complement of the Q output at all times.