Every connector, including video, should send power both ways
I've written a lot about how to do better power connectors for all our devices, and the quest for universal DC and AC power plugs that negotiate the power delivered with a digital protocol.
While I've mostly been interested in some way of standardizing power plugs (at least within a given current range, and possibly even beyond) today I was thinking we might want to go further, and make it possible for almost every connector we use to also deliver or receive power.
I came to this realization plugging my laptop into a projector which we generally do with a VGA or DVI cable these days. While there are some rare battery powered ones, almost all projectors are high power devices with plenty of power available. Yet I need to plug my laptop into its own power supply while I am doing the video. Why not allow the projector to send power to me down the video cable? Indeed, why not allow any desktop display to power a laptop plugged into it?
As you may know, a Power-over-ethernet (PoE) standard exists to provide up to 13 watts over an ordinary ethernet connector, and is commonly used to power switches, wireless access points and VoIP phones.
In all the systems I have described, all but the simplest devices would connect and one or both would provide an initial very low current +5vdc offering that is enough to power only the power negotiation chip. The two ends would then negotiate the real power offering -- what voltage, how many amps, how many watt-hours are needed or available etc. And what wires to send the power on for special connectors.
An important part of the negotiation would be to understand the needs of devices and their batteries. In many cases, a power source may only offer enough power to run a device but not charge its battery. Many laptops will run on only 10 watts, normally, and less with the screen off, but their power supplies will be much larger in order to deal with the laptop under full load and the charging of a fully discharged battery. A device's charging system will have to know to not charge the battery at all in low power situations, or to just offer it minimal power for very slow charging. An ethernet cable offering 13 watts might well tell the laptop that it will need to go to its own battery if the CPU goes into high usage mode. A laptop drawing an average of 13 watts (not including battery charging) could run forever with the battery providing for peaks and absorbing valleys.
Now a VGA or DVI cable, though it has thin wires, has many of them, and at 48 volts could actually deliver plenty of power to a laptop. And thus no need to power the laptop when on a projector or monitor. Indeed, one could imagine a laptop that uses this as its primary power jack, with the power plug having a VGA male and female on it to power the laptop.
I think it is important that these protocols go both directions. There will be times when the situation is reversed, when it would be very nice to be able to power low power displays over the video cable and avoid having to plug them in. With the negotiation system, the components could report when this will work and when it won't. (If the display can do a low power mode it can display a message about needing more juice.) Tiny portable projectors could also get their power this way if a laptop will offer it.
Of course, this approach can apply everywhere, not just video cables and ethernet cables, though they are prime candidates. USB of course is already power+data, though it has an official master/slave hierarchy and thus does not go both directions. It's not out of the question to even see a power protocol on headphone cables, RF cables, speaker cables and more. (Though there is an argument that for headphones and microphones there should just be a switch to USB and its cousins.)
Laptops have tried to amalgamate their cables before, through the use of docking stations. The problem was these stations were all custom to the laptop, and often priced quite expensively. As a result, many prefer the simple USB docking station, which can provide USB, wired ethernet, keyboard, mouse, and even slowish video through one wire -- all standardized and usable with any laptop. However, it doesn't provide power because of the way USB works. Today our video cables are our highest bandwidth connector on most devices, and as such they can't be easily replaced by lower bandwidth ones, so throwing power through them makes sense, and even throwing a USB data bus for everything else might well make a lot of sense too. This would bring us back to having just a single connector to plug in. (It creates a security problem, however, as you should not just a randomly plugged in device to act as an input such as a keyboard or drive, as such a device could take over your computer if somebody has hacked it to do so.)