Universal Serial Bus (USB) The Universal Serial Bus is a cable bus that allows high- Mbits/sec or 12 Mbits/sec, over a maximum cable length speed data exchange between a host computer and a variety of 5 meters. USB cable contains 4 wires: of peripherals. Supporting both attachment and removal of peripherals while the host system is running, USB was ■ Two of the wires form a twisted pair, which is used for dif- designed to provide a convenient and universal interface for ferential data transmission connecting peripherals to desktop computers, without the ■ The remaining two wires provide power and ground for de- need to power down the computer. In this article Barbara vices that don’t have their own 5 V DC power supply. provides an introduction to the USB standard, outlining the goals of the USB committee and describes the physical USB device speeds cabling and connectors, the physical and logical bus topol- The USB specification classifies USB devices into two main ogies, the types of data transfers, and the communication flow types: between the different layers of USB software. ■ Low-speed devices ■ Full-speed devices he Universal Serial Bus (USB) was originally designed by a consortium of telecom companies and desktop PC Low-speed devices Tsuppliers, which included Compaq, DEC, IBM, Intel, Low-speed USB devices (typically switches, mice and joysticks, Microsoft, NEC and Northern Telecom. Their primary purpose and keyboards) transfer USB data at 1.5 Mbits/sec. The cost of for working together was to provide a computer-telephony these devices is kept down by the fact that their low-bandwidth connection mechanism. cables can be designed without shielding. However, the members soon realized that the USB was well Full-speed devices suited to many applications outside the area of computer- Full-speed USB devices (such as video, disk and LAN telephony integration. interfaces) transfer data at 12 Mbits/sec. As might be expected, these full-speed devices are designed for performance, and their The goals of the USB committee were threefold: support of this higher data transfer rate is more expensive. ■ To provide an architecture that could link desktop PCs to the telephone network USB usage in 1998 ■ To develop an architecture that truly offered an easy way for users to connect and reconfigure peripherals ■ To provide a means by which a large number of peripherals could be attached to desktop PCs, without the traditional tangle of cables and different I/O interface standards. USB The USB connectivity vision Despite the fact that USB is well defined, desktop computer suppliers won’t switch over to this new peripheral standard Mouse Printer Modem Video LAN immediately. Most vendors plan to take a phased approach, by providing one (or possibly two) USB ports on their desktop Keyboard Serial Audio SCSI system, while still providing the traditional PC I/O interface ports. (See Figure 1) This transitional phase will allow time for USB usage in 200X both hardware and software companies to prepare for the shift to the USB architecture. USB USB USB While there are several other competing I/O architectures, USB will probably become the dominant desktop I/O architecture for Keyboard Mouse several reasons. (See the sidebar entitled The competition on Video Printer page 54.) LAN Scanner Digital audio I/O The USB physical media Figure 1 Digital telephony USB supports up to 127 devices, with bandwidths of 1.5 Modem Reprinted from VMEbus Systems / June 1998 / 1 Copyright 1998 ~ All rights reserved Universal Serial Bus (USB) USB physical topology ■ self-powered hubs The USB architecture supports a hierarchical topology, where ■ low bus-powered devices “downstream” hubs and devices are connected to “upstream” ■ high bus-powered devices hubs (in a star-like pattern) with the restriction of a maximum ■ self-powered devices of 5 hubs along any given chain. Figure 2 shows the tiers in this hierarchy. Hubs are intelligent devices that not only act as signal DC power flows downstream from the root hub to the various repeaters and routers, but also provide some basic power hubs and peripherals [4]. The maximum power that any device management [3]. can draw from the upstream bus is 500 mA @ 5V, with an added The root hub (in the USB host controller) is the anchor point Host (Root Hub) for all of the USB ports in the system. Data flows between the Host (Root Tier) root hub and the USB devices through the signal lines and hubs Upstream of the hierarchy in 2 directions: Tier 1 Hub 1 ■ downstream, starting at the root hub, and flowing from hub Tier 2 to hub, until it ultimately flows into a peripheral device Node ■ upstream, starting from a peripheral device, and flowing Hub 2 Node from hub to hub, until it reaches the root hub Tier 3 Downstream Hub 3 Hub 4 Node Node USB power distribution Tier 4 Each USB hub or peripheral might have its own local power supply, or it might draw its power from the bus, through its upstream cable. USB devices can be classified into 5 categories, Node Node Node Node based on how they derive their power: ■ bus-powered hubs Figure 2 The competition There are several I/O architectures that are competing with Mbits/sec over a maximum cable length of 4 feet [1]. However, USB to provide peripheral interconnection to desktop GeoPort was designed strictly to support telephony devices computers, including: and applications, and has never been extended to support other types of devices outside of this category. [2]. ■ Apple Desktop Bus ■ Geoport Access.bus ■ Access.bus Access.bus uses: ■ Firewire (IEEE1394) ■ a hardware layer based on the Phillips I2C bus [8] Apple Desktop Bus ■ a software layer developed by Digital, to provide additional The Apple Desktop Bus (ADB) is a low-speed serial bus that functionality for I2C connects input devices, such as keyboards, mouse devices and graphics tablets to a Macintosh computer. ADB provides a I2C bus is an acronym for Inter-IC bus. That name explains its simple read/write protocol for up to 16 devices. ADB supports purpose, which is to provide a communication link between data rates of up to 90 Kbits/sec. Some disadvantages of ADB integrated circuits. This bus provides 2 active wires, and a single are its relatively low data rate, and the fact that it is not an ground connection. The active wires are the Serial Data line and entirely “open” standard because Apple Computer owns the Serial Clock line, which are both bi-directional [8]. patents on ADB. Access.bus is an extension of I2C that routes the Serial Data GeoPort and Serial Clock lines through cables, to support keyboards, GeoPort has historically been an Apple proprietary interface. mice, monitors and microcontrollers. It provides speeds up to However, Apple is currently licensing the GeoPort spec- 100 Kbits/sec, and the protocols provided by its software layer ification to other companies. Apple Computer developed this allow it to interconnect up to 127 I/O devices. It is royalty free, interface to enable Macintosh telephony applications, but cable lengths are very limited, and there are no lines in the including both voice and data. cable for distribution of DC power to devices. Apple’s GeoPort Telecom Adapter provides hardware and Firewire (IEEE1394) software that supports: Firewire (IEEE1394) supports up to 63 peripheral devices, and offers very high data transfer rates – from 100 Mbits/sec to 400 ■ data communications Mbits/sec. It provides ample bandwidth to support almost any ■ fax communications peripheral, and is well suited to I/O applications that require ■ telephony services such as voice mail very high-speed data transfers. However, its complexity (and its high cost) make it impractical as a desktop I/O interconnect The standard GeoPort uses RS-422 signaling, and provides a for low-performance, low-cost peripherals, such as mice and point-to-point link that supports data transfer rates up to 2 keyboards. [2] 2 / Reprinted from VMEbus Systems / June 1998 Copyright 1998 ~ All rights reserved restriction that the maximum current that it can draw during The logical USB architecture startup is 100 mA. Figure 4 shows the logical topology of the USB architecture shown in Figure 2. While the devices are physically attached to A self-powered hub can be designed with a power supply that the USB in a tiered, star topology, the details of this physical provides the maximum DC current (500 mA @ 5V) to each of hierarchy are hidden from the application program by the system its downstream ports. However, a bus-powered hub has only the software, which presents each logical device to the application DC power from the upstream cable (500 ma @ 5V) to distribute program as a collection of endpoints. The application program to all of its downstream ports. If several devices are connected can then exchange data directly with each endpoint. [1]. This to its downstream ports, the bus-powered hub might draw more logical connection to each device endpoint is represented by a than its allotted 500 mA. point-to-point line segment in Figure 4. To prevent damage to the DC power supplies, the host (root hub) Hub 1 and all self-powered hubs must have overcurrent protection on Node Node their DC power supplies. If any self-powered hub detects an Node Hub 4 aggregate current drawn by all downstream ports in excess of 5.0 Amps, its overcurrent protector removes power from all downstream ports, and it reports the condition to the host controller [1]. Hub 2 Application Node program USB cable connectors Some USB devices (such as keyboards, mice and hubs) have Node their upstream cable permanently attached to them.