A+ IT Foundations A USB is managed by a host controller. Each host controller supports multiple ports attached to the same bus. In theory, there could be up to 127 connected devices per controller, but to overcome the limitations of sharing bandwidth, most PC motherboards provision multiple USB controllers, each of which has three or four ports. USB Standards There have been several iterations of the USB standard. Each version introduces better data rates. A version update may also define new connector form factors and other improvements. The USB 2.0HighSpeed standard specifies a data rate of 480 Mbps shared between all devices attached to the same host controller. The bus is half-duplex, meaning that each device can send or receive, but not at the same time. Iterations of USB 3.x introduced new connector form factors and upgraded transfer rates, each of which are full-duplex, so a device can send and receive simultaneously. USB 3.2 deprecated some of the older terms used to describe the supported transfer rate: Standard USB 3.2 Gen 1 SuperSpeed USB Speed 5 Gbps Connectors USB-A, USB-C, USB Micro USB 3.2 Gen 2x1 SuperSpeed USB 10 Gbps USB 3.0 USB 3.1 10 Gbps USB-A, USB-C, USB Micro 2 x 10 Gbps USB-C USB 3.2 Gen 2x2 SuperSpeed USB 20 Gbps Legacy Desi USB 3 controllers feature two sub-controllers. One controller handles SuperSpeedcapable devices, while the other supports legacy HighSpeed, FullSpeed, and LowSpeed USB v1.1 and v2.0 devices. Consequently, legacy devices will not slow down SuperSpeed-capable devices. USB Connector Types The connector form factors specified in USB 2 are as follows: SuperSpeed+ Type A—For connection to the host and some types of peripheral device. The connector and port are shaped like flat rectangles. The connector should be inserted with the USB symbol facing up. Type B—For connection to large devices such as printers. The connector and port are square, with a beveled top. Type B Mini—A smaller peripheral device connector. This type of connector was seen on early digital cameras but is no longer widely used. Type B Micro—An updated connector for smaller devices, such as smartphones and tablets. The micro connector is distinctively flatter than the older mini type of connector. USB 2 ports and connectors. (Image © 123RF.com) A USB cable can feature Type A to Type A connectors or can convert from one type to another (Type A to Type B or Type A to Micro Type B, for instance). In USB 3, there are new versions of the Type A, Type B, and Type B Micro connectors with additional signaling pins and wires. USB 3 receptacles and connectors often have a blue connector tab or housing to distinguish them. USB 3 Type A connections are physically compatible with USB 1.1 and 2.0 connections, but the Type B/Type B Micro connections are not. So, for example, you could plug a USB 2 Type A cable into a USB 3 Type A port, but you could not plug a USB 3 Type B cable into a USB 2 Type B port. USB 3 connectors and ports (from left to right): Type A, Type B, Micro Type B, Type C. (Image ©123RF.com) USB 3.1 defines the USB-C connector type. This compact form factor is intended to provide a single, consistent hardware interface for the standard. The connector is reversible, meaning it can be inserted either way up. The connector design is also more robust than the earlier miniB and microB types. USB-C can use the same type of connector at both ends, or you can obtain USB-C to USB Type A or Type B converter cables. Cable Length The maximum cable length for LowSpeed devices is 3 meters, while for FullSpeed and HighSpeed the limit is 5 meters. Vendors may provide longer cables, however, results might be limited past the max recommended length. Although SuperSpeed-capable cables do not have an official maximum length, up to about 3 meters is recommended. Power As well as a data signal, the bus can supply power to the connected device. Most USB Type A and Type C ports can be used to charge the battery in a connected device. Basic USB ports can supply up to about 4.5 watts, depending on the version. A power delivery (PD)–capable port can supply up to 100 watts, given suitable connectors and cabling.
