SR latch, S = 1, R = 1 — flip-flops
introthe forbidden input
Answer
Q⁺ = 0, Q̄⁺ = 0
Why this example is worth doing
Setting and resetting at once breaks the invariant that Q and Q̄ are opposites, and on release the outcome depends on which input falls first — a race with no logical answer. The page marks the row forbidden rather than don't-care, and notes that this defect is precisely what the JK flip-flop was invented to remove, which motivates the rest of the page.
Try your own input in the Flip-flops & latches. SR, D, JK and T: characteristic tables, excitation tables and timing diagrams.
How the answer is reached
The probed row
S = 1, R = 1, Q = 0, Qn = 1
Output table
| S | R | Q | Qn | Q⁺ | Q̄⁺ |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 |
| 0 | 0 | 0 | 1 | 0 | 1 |
| 0 | 0 | 1 | 0 | 1 | 0 |
| 0 | 0 | 1 | 1 | 0 | 0 |
| 0 | 1 | 0 | 0 | 0 | 1 |
| 0 | 1 | 0 | 1 | 0 | 1 |
| 0 | 1 | 1 | 0 | 0 | 0 |
| 0 | 1 | 1 | 1 | 0 | 0 |
| 1 | 0 | 0 | 0 | 1 | 0 |
| 1 | 0 | 0 | 1 | 0 | 0 |
| 1 | 0 | 1 | 0 | 1 | 0 |
| 1 | 0 | 1 | 1 | 0 | 0 |
| 1 | 1 | 0 | 0 | 0 | 0 |
| 1 | 1 | 0 | 1 | 0 | 0 |
| 1 | 1 | 1 | 0 | 0 | 0 |
| 1 | 1 | 1 | 1 | 0 | 0 |