Q: What is the transmission line structure of USB?

Answer: A USB transmission line is composed of four lines: ground wire, power wire, D+, and D -. D+and D - are differential input lines, which use a voltage of 3.3V (note that it is different from the 5V level of CMOS). The power wire and ground wire can provide a voltage of 5V to the device, with a maximum current of 500MA (which can be set in programming, as for the hardware implementation mechanism, don't worry about it).

Q: How is data transmitted on a USB transmission line?

Answer: Data is transmitted on a USB cable from low to high.

Q: What is the encoding scheme for USB data cables?

Answer: USB uses non return to zero inversion to transmit data. When the differential data on the transmission line is inputted as 0, it is inverted, and when inputted as 1, it remains the original value. To ensure the accuracy of signal transmission, when a packet is sent on the USB bus, the transmission device needs to perform a bit insertion operation (that is, after every 6 consecutive 1s in the data stream, a 0 is inserted), forcing the NRZI code to change.

Q: What is the data line format for USB?

Answer: Like everything else, USB data is composed of binary digit strings. Firstly, the digit strings form domains (there are seven types), domains then packets, packets then transactions (IN, OUT, SETUP), and finally transactions form transfers (interrupt transfers, parallel transfers, batch transfers, and control transfers).