I have an iPhone, iPad, and a Mac and when I connect my phone or iPad to the Mac, it automatically starts charging. But why? Why not the other way around?
My thinking is that Mac’s have a bigger battery? But still what about that makes the Mac a charging source in the presence of an iPhone/ iPad?
In: Technology
Before USB, there was no real standard for what plugs and ports each different device had outside of keyboards/mice. So since there were so many unique devices out there that all needed to connect to computers, there was a standard developed called USB that would allow devices that needed to connect to a computer to have one port known as USB-B, and the computer would have the other side known as USB-A. The USB-B port is commonly referred to as a “Printer” port since that’s where they were first used.
The device that had the USB-A port would be the “Host”. It would supply 5 volts of power to “charge” any device that was connected, and it would also be the device that was in control of the USB connection. The device that had the USB-B port would be the “client” device that wasn’t in control, all it was supposed to do was accept power and accept commands from the other device. This worked for a REALLY long time.
But USB-B is kind of a big plug, and devices were getting smaller and smaller, so we needed something else. Then they brought about “Mini USB-B”. This was a slightly smaller plug, but things still worked in much the same way as before — The device with the USB-A port was in charge, and the device with the Mini USB-B port was supposed to accept power and commands.
But devices KEPT getting smaller, so they came up with “Micro USB-B”. And for a while, this worked flawlessly as well; It was just a smaller version of Mini USB-B. The device with the USB-A port was still in charge. But then things started changing and devices that were connecting to USB started getting smarter.
Eventually someone said “Hey, I have a portable device that’s basically a fully-fledged computer now. I want to connect a keyboard, a mouse, a flash drive, a game controller, etc. to it and still be able to use it” so they invented a new standard called USB On-The-Go or USB-OTG. It used the same Micro USB-B port as before, but with a special cable it could be adapted to a USB-A port and act as if it were a host device that would normally only have a USB-A port on it.
“Well, that’s really confusing”, said someone, “We need a new port that can do both things if it’s going to be like this”. So they invented a new standard. Because it was both USB-A and USB-B at the same time, they decided to call it “USB-C”. And that just stuck. New devices that could only act as a host could have USB-A ports, but devices that could act as both a host and a client could have a USB-C port. Eventually, the USB-C port became standard on ALL devices, not just ones that could do both, so there needed to be a new solution: Let the devices themselves figure it out. So this is where we are now.
How do devices figure out what to do? Devices that can act as both a host and a client have a chip inside of them. When they’re connected to another device, these chips communicate with one another over the USB-C cable to work out which device should do what. The chip in your phone knows that it’s a phone, so it will request power instead of send power most of the time. If there’s a device that needs power, it will work it out. Equally, the chip inside your laptop knows that it’s a laptop so it will send power instead of requesting power most of the time. If the laptop is receiving power, it will work it out.
In situations where both devices can both send and receive power, like if you connect two laptops or two phones together? Well, that’s not common so there’s no real standard so anything can happen, honestly. The chips inside each device will work it out. The end result is that one laptop or one phone will charge the other, but which of the two things happens is almost entirely random.
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