GATE EC 2015 Set 3 — Question 48

MCQ+2 / -0.67MediumSR & D LatchesSequential LogicDigital Circuits

Digital Circuits → Sequential Logic → SR & D Latches

Last updated 5 September 2026

Question

An SR latch is implemented using TTL gates as shown in the figure. The set and reset pulse inputs are provided using the push-button switches. It is observed that the circuit fails to work as desired. The SR latch can be made functional by changing

Correct answer (D) 5 V to ground Solution In the given circuit, the push buttons are connected to 5V. In TTL logic, an open input typically floats HIGH. 1. When the button is open, the input to the inverter is HIGH, so the inverter output is LOW. 2. When the button is pressed, the input to the inverter is connected to 5V (HIGH), so the inverter output remains LOW. Because the logic level at the inverter input never changes from HIGH, the latch inputs stay at 0, and the latch cannot be set or reset. By changing the 5 V supply to Ground : 1. When the button is open, the input floats HIGH (TTL characteristic), and the inverter output is LOW. 2. When the button is pressed, the input is pulled to Ground (LOW), and the inverter output becomes HIGH. This allows the NOR-based SR latch to receive active-high pulses for Set and Reset operations, making it functional. Turn this into a strength. Explore AI-powered practice and doubt support with Success Tracker.

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Correct answer (D) 5 V to ground Solution In the given circuit, the push buttons are connected to 5V. In TTL logic, an open input typically floats HIGH. 1. When the button is open, the input to the inverter is HIGH, so the inverter output is LOW. 2. When the button is pressed, the input to the inverter is connected to 5V (HIGH), so the inverter output remains LOW. Because the logic level at the inverter input never changes from HIGH, the latch inputs stay at 0, and the latch cannot be set or reset. By changing the 5 V supply to Ground : 1. When the button is open, the input floats HIGH (TTL characteristic), and the inverter output is LOW. 2. When the button is pressed, the input is pulled to Ground (LOW), and the inverter output becomes HIGH. This allows the NOR-based SR latch to receive active-high pulses for Set and Reset operations, making it functional.

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