Reading schematics and datasheets: the basics

Wires and junctions, net labels, power and ground symbols, off-page ports, pin numbers versus pin names, active-low notation, and what to read first in a datasheet — explained for first-time readers.

Last updated: 2026-10-09

A schematic is a map of what connects to what. Where parts physically sit is the PCB layout's job; the schematic shows only connections and values. This article covers the conventions a first-time reader needs. Keep the example schematic open and look for each item as you read.

Wires and junctions

• Wire: a line between part pins. Every pin joined by wires is electrically the same point; that group is a net (or node).
• Junction dot: a solid dot where lines meet means "connected here".
• Crossing without a dot: two lines crossing with no dot are not connected. Older drawings hop one line over the other with a small arc. To avoid ambiguity, modern practice dots T-junctions and avoids four-way joins.

Clicking a wire in Schematic Viewer highlights the whole net, so you can confirm at once that a dotless crossing really isn't connected.

Net labels: connecting without wires

Drawing every connection as a line would turn the sheet into a spider's web. Instead a wire ends in a net label such as SDA or ESP_EN, and labels with the same name are connected even with no line between them. Hunting for every label with the same name is the slowest part of reading a schematic — exactly what Schematic Viewer does for you.

Names usually describe function. Names with _N, # or / are often active-low (see below), and TX/RX may be named from either side of the link, so check them against the pin names.

Power and ground symbols

• Power symbols: an upward arrow or bar with a name such as +3.3V, VCC or +5V. As with net labels, the same name means the same net everywhere.
• Ground symbols: stacked lines getting shorter (GND), a triangle, or a rake. Some designs separate analog (AGND), digital (DGND) and chassis ground; different names are different nets, usually joined at one point through a bead or 0 Ω resistor.
• Look-alike names: +3.3V and +3.3VA, or VCC and VCCIO, are different supplies split off through a filter or switch. Know which one you are probing.

Connections across pages: ports and hierarchical sheets

Multi-page schematics pass signals between pages with off-page connectors — a name inside an arrow or pentagon. The same name means connected, and the other page number is sometimes printed beside it.

Large designs use hierarchical sheets: a top page shows blocks such as "Power" or "MCU", and ports on a block's border connect to ports on its sub-page. Here the same name can be local to each block, so take care. Schematic Viewer joins named labels and ports across pages, but does not yet follow connections made only through unnamed block ports (see Supported files).

Symbols and pins

• Pin numbers and pin names: on an IC symbol the pin number (physical position, e.g. 12) is usually outside the body and the pin name (function, e.g. GPIO4) inside. Probe and solder by number; understand by name.
• Polarized parts: on a diode the bar the triangle points to is the cathode. Electrolytic capacitors show polarity with a + or a curved plate.
• Hidden pins: some libraries leave an IC's power and ground pins off the symbol and connect them by name. If you can't see a VCC pin, check the datasheet.

Active-low notation

A signal that acts when it is at 0 V (low) is marked with a bar over its name or with /, #, _N or n. RESET_N, for example, resets the chip when low, so a pull-up resistor usually holds it high. The overbar is sometimes lost when exporting to PDF, so the pull-up or pull-down arrangement is the safer clue.

What to read first in a datasheet

When you meet an unfamiliar IC, look up its part number and read in this order:

1. Features on page one: what the chip does, its supply range and headline specs.
2. Pin configuration and pin description table: match the schematic's pin numbers and names. Pinouts differ between packages, so use the drawing for the package on your schematic.
3. Absolute maximum ratings: the limits beyond which damage occurs — not to be confused with the recommended operating conditions.
4. Electrical characteristics: the reference for your measurements; note the test conditions (voltage, temperature).
5. Typical application: the maker's recommended circuit. Most surrounding capacitor and resistor values on a schematic come from here, so comparing the two explains each part's job.

A suggested reading order

For an unfamiliar schematic, follow ① the power input through the regulators to each supply, ② the main IC (an MCU, say) with its clock and reset, then ③ the signals entering and leaving through connectors. At each step, click the supply or signal name in Schematic Viewer's list to see how far it reaches. Reference designators and value notation are covered in Finding parts.