usbc-cable-caps 0.1.0

Decode what a USB-C cable can actually carry — power, data rate and video — from its e-marker class rather than its label.
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# usbc-cable-caps

Every USB-C plug is the same shape. Almost nothing else about USB-C cables is.

```toml
[dependencies]
usbc-cable-caps = "0.1"
```

```rust
use usbc_cable_caps::*;

let bundled = typical_bundled();
bundled.max_watts_at(20.0);            // 60 W — not the 100 W on the charger
bundled.data.carries_video();          // false

let marked = Cable { emarker: true, declared_amps: 5.0, ..typical_bundled() };
marked.can_deliver(100.0, 20.0);       // true
```

## Two facts decide everything

**Above 3 A a cable needs an e-marker chip.** The chip declares the rating to the
source. Without it the source must assume 3 A, and it will — no matter how thick
the copper inside actually is. This is a protocol limit, not a thermal one, and
it is why a well-built unmarked cable still caps a 100 W laptop charger at 60 W.

**Power and data are independent axes.** A cable can carry 240 W and 480 Mbps.
Another carries 60 W and 40 Gbps. The connector reveals neither. The most common
disappointment follows directly: a cable that charges a laptop flawlessly and
shows nothing at all on an external monitor, because USB 2.0 wiring has no
high-speed pairs for the DisplayPort alternate mode to use.

## Why long fast cables cost what they do

Signal loss rises with both length and frequency, so the faster the class the
shorter a passive cable can be — roughly 4 m at USB 2.0 speeds, under a metre at
40 Gbps. Longer fast cables exist, but they contain active signal conditioning.
`passive_length_is_plausible` encodes that relationship, and it doubles as a
sanity check on listings: a cheap passive 3 m cable advertising 40 Gbps is
claiming something the physics does not allow.

## On the naming

USB 3.0 became USB 3.1 Gen 1, then USB 3.2 Gen 1, all describing the same 5 Gbps.
This crate uses `Gen1`, `Gen2`, `Gen2x2` and `Usb4` because the throughput is the
only stable thing about the naming. Treat any speed claim that cites a version
number rather than a bitrate as unresolved.

Specifications for individual cables — gauge, e-marker rating and data class —
are listed on [CairoVolt's cable pages](https://cairovolt.com/en/cables).

## Licence

MIT