Digital logic design interviews tend to test the same handful of things repeatedly, across four distinct question shapes. Knowing the shape you're being asked matters as much as knowing the material.
Conceptual checks
"What's a Karnaugh map for?" "Why minimize a Boolean expression at all?" "What's the difference between SOP and POS?" These aren't testing whether you can recite a definition — they're testing whether you actually understand why the technique exists, not just how to run it. See What Is a Karnaugh Map? and SOP vs. POS Explained for the reasoning worth being able to explain in your own words, not memorized verbatim.
"Design this circuit" prompts
A live version of a textbook problem — design a majority voter, a parity checker, a comparator — asked on a whiteboard, sometimes with a twist (extra don't-cares, an added output). The worked examples on this site cover the most common versions of this directly: full adder, parity checker, magnitude comparator. Being able to derive the truth table from the verbal description quickly, not just recognizing the final answer, is what's actually being watched.
"Explain the trade-off" questions
Less about getting a specific numeric answer, more about demonstrating judgment: "when would you use POS instead of SOP?", "what's the cost of a multi-level circuit versus two-level?", "why would a real chip design tool use a heuristic instead of an exact method?" These map directly onto Two-Level vs. Multi-Level Logic Circuits and The Espresso Heuristic Logic Minimizer — the honest answer to each is "it depends on what you're optimizing for," and being able to name what that trade-off actually is matters more than picking a side.
The genuinely tricky one: the cyclic case
Occasionally an interviewer deliberately picks a function with no essential prime implicants at all — a cyclic chart, covered fully (with a real verified example) in Petrick's Method Explained. The tell that you're looking at one: every candidate group covers the same number of cells as every other, and none of them is uniquely required anywhere. Recognizing that shape on sight, rather than getting stuck trying to find an essential group that doesn't exist, is exactly the kind of thing this question is designed to surface.
What's actually being evaluated
Across all four question shapes, the same thing is really being checked: whether the reasoning holds up under a follow-up question, not whether the first answer sounded confident. Being able to explain why a group is essential, why a don't-care was safe to use, or why a specific trade-off applies is what separates a memorized answer from real understanding — and it's exactly the kind of reasoning the solver's Prime Implicant Analysis panel makes visible directly, which is worth practicing against before an actual interview.