1 Overview

The Universal Serial Bus (USB) is an industry standard that specifies connection, communication, and power supply between host systems and peripherals. USB has evolved over the years through a series of standards focusing on increased data rates. The following table breaks down the standards including naming convention, data lines, nominal rates, and connector types.

There are two types of data pairs: half-duplex (HDx) and full-duplex (FDx). USB 2.0 and earlier standards use a single half-duplex which provides communication in both directions but only one direction at a time. A half-duplex translates to the D+ and D- data lines. USB 3.0 and later implement a single half-duplex (D+, D-) for USB 2.0 compatibility and two or four pairs in full-duplex with the full-duplex allowing for bidirectional communication simultaneously. A pair is considered to be either the transmit lines (TX+, TX-) or the receive lines (RX+, RX-). USB 3.0 and later include at least two pairs (TX+, TX-, RX+, RX-), also known as a lane.

Since there are many USB standards, determining the data rates can be confusing specifically for USB 3.2 and USB 4. The general consensus is that if the format is AxB then the last digit determines the number of lanes. For example, USB 3.2 Gen 2x2 has a nominal rate of 20Gbps, but since there is a 2 as the last digit, this means there are 2 data lanes each with 10Gbps totaling 20Gbps.

The following sections go into detail on ESD protection based on the nominal rate for USB interfaces.