USB 1.x Electrical Interface
With all of its sophisticated error checking and correcting, a USB cable wouldn’t be of much use if its electrical signals were not up to scratch. So a lot of work has been done to make sure that the transfer of data is reliable by paying attention to the cable construction, its shielding and physical characteristics. Of course, if you are a normal user, all you have to do is use compliant cables. But if you are involved in creating USB devices, you probably want to know more.
We will just talk about the original USB 1.x low and full speed signaling. The rise and fall times are different for each speeds. The signal transmitters and receivers are also different for each speed. Interestingly enough, the USB cable must be able to withstand the shorting of any of the two signal lines without damage to the receiver and transmitter hardware on each line.
An electrical path between a USB device and an “upstream” hub is called a segment. The upstream term is used because USB is organized as a tree with the desktop computer host as the root. This means that you can also have a hub connected to an “upstream” hub on a segment. The lowest segment determines the data rate to the desktop computer host. A low speed data rate can be carried on a higher speed segment, it’s just at a lower rate. You can’t have a high-speed rate carried by a low speed segment.
There are several electrical differences between low and full speed. First is the rate. Low speed is 1.5 Mbs and full speed is 12 Mbs. Another difference is that low speed is inverted compared to full speed. That is a logic “1” has its voltage reversed on the USB D+ and D- lines (data) compared to a “1” on full speed. Thirdly, The slow speed has a longer rise time on its pulse’s edges. This allows for less signal reflection on the cable and allows you to have thinner, cheaper cables without shielding like you see with keyboards and mice.
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