Termination

A bus long enough to matter is a transmission line, and a signal reaching the end of an unterminated line does not simply stop. It reflects, travels back down the line, and collides with whatever is following it.

A terminator is a resistor across the line at that end, sized to the line’s characteristic impedance, which absorbs the signal instead.

flowchart TB
    subgraph U["unterminated"]
        direction LR
        A["driver"] --> B["far end,<br/>nothing across the line"] --> C["the signal reflects and<br/>collides with what follows"]
    end
    subgraph T["terminated"]
        direction LR
        A2["driver"] --> B2["far end,<br/>120Ω bridging the pair"] --> C2["the resistor absorbs it"]
    end
    U ~~~ T

The value is not a choice. It is the impedance the bus standard specifies, which is why CAN terminators are 120Ω on every board that has ever carried CAN. On a differential pair the resistor bridges the two halves; on a single-ended bus it goes to ground.

A profile declares the requirement rather than the board proving it, since a netlist cannot tell a terminator from any other resistor of that value. See profile-termination.

Where the course teaches it: chapter 1 for the resistor, chapter 12 for why the copper decides.