OR from NOR gates only — OR gate
exam standarduniversal-gate construction
Answer
2 NOR gates realising B + A
Why this example is worth doing
The NOR-only construction of OR mirrors the NAND-only construction of AND exactly: compute the inverted function, then invert it back with a second gate whose inputs are tied. Presenting the two side by side is the clearest way to show that NAND-world and NOR-world are the same world under duality, and that anything provable in one has a mechanical translation into the other. The universal gates page takes this up systematically.
Try your own input in the OR gate. Truth table, symbol and algebraic form for A + B, with a live two-input toggle.
How the answer is reached
Gate list
| Node | Gate | Inputs |
|---|---|---|
| g1 | nor | A, B |
| g2 | nor | g1, g1 |
NOR-only realisation
A ↓ B ↓ (A ↓ B)B + A— 2 NOR gates, verified by reading the network back out.
Truth table
| # | A | B | A ↓ B ↓ (A ↓ B) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 1 | 0 | 1 | 1 |
| 2 | 1 | 0 | 1 |
| 3 | 1 | 1 | 1 |