I got to thinking about USB’s tonight, and I know what they do, but I don’t know how they work. Serial cables have pins, and each pin is designated with a certain function, but USB’s are different in that they don’t have pins. So how do they work, exactly? Also, how much power can a USB cable carry and how much power can the port they plug into handle?
There are 4 pins in a USB connector. 5V, ground, data+ and data-. A USB port can source upto 500mA. At 5V that is 2.5 Watts.
There is big document at www.usb.org which explains in great detail exactally what happens.
Yeah, it’s not that they don’t have pins - they do. They’re just not the same sort of pins as the old-style “D-sub” connectors (9 or 25-pin RS-232 serial connectors of old). If you look in the end of a USB connector you can plainly see 4 contacts, though.
What’s really cool about USB is that it only needs 2 data pins to do everything it does. The difference between the old-style RS-232 serial and USB is like the difference between a tricycle and a Cadillac.