# Reading a multi-gate schematic

*From one gate to a block*

A multi-gate circuit is built by feeding the output of one gate into the input of another, and reading its schematic means tracing each wire (net) from the single source that drives it to every gate input that consumes it.

Group: Gates
URL: https://digiwleea.wleeaf.dev/learn/combining-gates/

> **KEY:** So far each lesson has shown one gate at a time: [AND](https://digiwleea.wleeaf.dev/learn/and/), [OR](https://digiwleea.wleeaf.dev/learn/or/), [NOT](https://digiwleea.wleeaf.dev/learn/not/). Real circuits chain many gates together, and the first time you see a schematic with a dozen of them it can look like a tangle. This bridge gives you the one habit that untangles any of them, so the [half adder](https://digiwleea.wleeaf.dev/learn/halfadder/) and every block after it reads cleanly.

The one idea: **a wire (a net) has exactly one source and any number of destinations.** Something drives it (a gate output, or an input pin), and one or more gate inputs listen to it. A gate's output is not the end of the story, it becomes the input to the next gate. That single feed-forward, output-into-input, is the only mechanism by which small gates become big blocks. There is no other trick.

So reading a schematic is just following signals. Start at the primary inputs on the left. For each gate, find the nets feeding its inputs (trace each back to whatever drives it), decide the gate's output from its truth table, and label that output net. Now that labelled net is an input to the gates further right. Repeat, left to right, and you have computed the whole circuit one gate at a time. A schematic is not read all at once; it is *walked*.

A worked micro-example: an AND gate whose output feeds one input of an OR gate, and the OR's other input is a signal `C`. Call the AND output `p`, so `p = A AND B`. Then the final output is `p OR C = (A AND B) OR C`. You did not need to see all three gates at once, you named the middle net `p` and the rest followed. Naming internal nets is the whole skill.

> **WARN:** Two wires that cross on the page are **not** connected unless there is a junction dot. Beginners often read a crossing as a join and merge two independent nets. A net is one electrical node: only wires actually tied together (a dot, or meeting at a pin) share a value. In this simulator a real connection is an unbroken path; a mere crossover is nothing.

**Q (Try it):** A NOT gate's output feeds one input of an AND gate; the AND's other input is `B`. If the NOT's input is `A`, write the Boolean expression for the AND's output, naming the internal net as you go.

**A:** Call the NOT's output `n`, so `n = NOT A`. The AND then outputs `n AND B = (NOT A) AND B`. By naming the middle net `n` you turn a two-gate schematic into one short expression, which is exactly how you read any multi-gate block: label each internal net, then combine left to right.

### FAQ

**Q:** How do you read a logic schematic?

**A:** Walk it left to right. Start at the inputs, and for each gate trace its input nets back to their sources, apply the gate's truth table, and label its output net. That output then feeds gates further right. Naming each internal net turns the whole diagram into a chain of simple steps.

**Q:** What is a net in a circuit?

**A:** A net is one electrical node: a single source (a gate output or input pin) and every gate input connected to it. All points on a net carry the same value.

**Q:** Do crossing wires in a schematic connect?

**A:** No, not unless there is a junction dot where they meet. Wires that merely cross on the page are separate nets. A connection requires the wires to actually be tied together.

> **KEY:** You can now read any block as a chain of single gates. Next, watch two gates side by side become the [half adder](https://digiwleea.wleeaf.dev/learn/halfadder/), the first circuit that does real arithmetic.