How do cables know that they are plugged in?


Recently I’ve been messing around with HDMI cables since I’ve been watching Netflix movies and TV shows on my TV that isn’t smart.

I have noticed that the cables can discern if they’re plugged in or not. If I plug in the cable with my computer off, the TV says “No signal”, and if I disconnect the cable, the TV says “Verify cable connection”. How do the device knows the difference?

I apologize for my poor English.

In: Technology

There is a specific wire inside the cable that is reserved for detecting whether the connectors are plugged in. Basically one half of a simple circuit is in the display and the other half is in the source device. When the two halves are connected via the cable, they output a different signal than what they do when they are separated.

A HDMI cable has something like 8 or 10 pairs of wires, 16 or 20 total connection points. Each one of those connection points runs a certain type of data.

For example two of the pins on and HDMI cord may be to run a bit of low voltage power through to let the device know that it’s connected and ready to go

It’s like shaking hands; if you hold out your hand to someone, you’re expecting a physical connection with the other person’s hand. If you’re not holding his hand yet you’re not “receiving a signal”. When your hands lock, you are. Same goes for cables. The output device, like a monitor or TV expects a signal from the connector your monitor/TV is set to (AV input). As long as nothing’s plugged in, it will show “no signal”.

HDMI devices receive 5v from the source device to power up circuitry in the display device (you can power a device from an HDMI port alone, e.g. Chromecasts, etc. from that 5v alone just by plugging it into the TV, though they often give you a USB cable too).

There’s also an enormous amount of handshaking where the two devices tell each other what they can and can’t do, like VGA monitors used to with EDID.

It’s all old technology, nothing to do with HDMI – many TVs and projectors can detect a signal on even a composite video cable by just the 1v peak-to-peak signal coming down it, and SCART (Euroconnector) is a decades-old predecessor of HDMI, basically, that can carry lots of different signals (audio, composite, RGB, S-Video, data, etc.) and also supplies power on a certain pin if one of those devices wants to show on the screen (which can trigger the TV to turn on, or change channel to that device).

Ethernet cables contain 4 pairs of wires. When the computer is plugged into the wall, its typical connecting directly to a switch that is in a data closet somewhere nearby (within 300Ft). The computer sends a signal down pair 4 (brown and white brown) to tell the switch that it is plugged in. Then negotiations start for the connection type (speed/duplex).