Classical SR flip-flops require two NAND gates for active-low inputs or two NOR gates for active-high inputs. The circuit shown uses only two inverters connected in a latch configuration via resistors R1 and R2 (see the figure). This arrangement is useful when a design requires an SR flip-flop, but there are only a few unused inverters, and no NAND or NOR gates are available.

Furthermore, this SR flip-flop topology is more versatile than traditional SR flip-flops. That's because it can behave either like a NAND-based or NOR-based flip-flop, depending on the polarities of diodes D1 and D2.

When diodes D1 and D2 are connected as shown in the figure, the inputs are activated by a positive pulse and the circuit behaves like a SR flip-flop built with NOR gates. In this case, A is the SET input and B is the RESET input.

If the polarities of D1 and D2 are reversed, the circuit acts like a classical NAND-based SR flip-flop with active low inputs. In this case, B is the SET input and A is the RESET input. Of course, you can add more diodes to increase the number of independent S and R inputs. The values of R1 and R2 aren't critical but they should be equal.

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Reader Comments I NEED A SIMPLE ELECTRONIC CIRCUIT MAYBE YOU LIKE TO DISPLAY ONE Anonymous -September 10, 2007 how to work sr flip-flop?define truth table basis? Anonymous -August 26, 2006 Yes a diagram is definately necessary. Maybe you could animate it so that it shows the bits going through each gate and the output from each gate for a better understanding of this type of circuit. xoSlantEyesxo -November 14, 2004 Quite interesting but you need to show proper working as well as the diagram. Amit -October 14, 2004
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