SR latch polarity (NAND version) In a cross-coupled NAND flip-flop (latch), the set and reset inputs are normally active-LOW. How can we modify the circuit so that it instead has active-HIGH S and R control inputs (i.e., asserting a HIGH performs set/reset)?

Introduction / Context:
NAND- and NOR-based cross-coupled latches implement the same memory function but with different input polarities. A standard NAND latch uses active-LOW S and R (bringing an input LOW asserts that function). Many systems, however, specify active-HIGH control signals. This question asks how to reconcile that by simple logic-level adaptation.

Given Data / Assumptions:

• Device: cross-coupled NAND SR latch.
• Default polarity: S and R are active-LOW.
• Goal: make S and R active-HIGH without changing the fundamental latch core.

Concept / Approach:
Polarity conversion is achieved by inversion. If a function is active-LOW and we require active-HIGH behavior, we place an inverter in series with each control line. A HIGH at the external interface becomes a LOW into the NAND input, thus producing the original active-LOW action inside the latch while presenting an active-HIGH interface externally.

Step-by-Step Solution:

Verification / Alternative check:
Truth-check one case: External S=1, R=0 should set Q. After inversion, internal S=0, R=1 into the original NAND latch, which indeed sets Q=1. All other cases map consistently.

Why Other Options Are Wrong:

• It can't be done: Incorrect—simple inversion is standard practice.
• Invert the Q outputs: Changes output polarity, not input control polarity.
• Insert RC networks: Affects timing/edges, not logic-level polarity.

Common Pitfalls:
Confusing active-level conventions, or attempting to swap NAND for NOR (which changes internal behavior rather than simply inverting inputs).

Final Answer:
Invert the S-R inputs.