Designing Embedded Hardware, 2nd Edition

Physical Interface

USB uses a shielded, four-wire cable to interconnect devices on the network (Table 11-2). Data transmission is accomplished over a differential twisted pair (much like RS-422/485) labeled D+ and D-. The other two wires are V _BUS, which carries power to USB devices, and GND. Devices that use USB power are known as bus-powered devices , while those with their own external power supply are known as self-powered devices . To avoid confusion, the wires within a USB cable are color-coded.

Table 11-2. USB wires

Connector pin	Signal	Purpose	Wire color
1	VBUS	USB device power (+5V)	Red
3	D+	Differential data line	Green
2	D-	Differential data line	White
4	GND	Power and signal ground	Black

Some USB chips refer to D+ and D- as DP and DM, respectively.

The connection from a device back to a host is known as an upstream connection. Similarly, connections from the host out to devices are known as downstream connections. Different connectors are used for upstream and downstream ports, with the specific intention of preventing loopback. The only way to connect a USB network is a tiered star. USB uses two types of plugs (jacks) and two types of receptacles (sockets) for cables and equipment. The first type is Series A, shown in Figure 11-6. Series A connectors are for upstream connections. In other words, a series A receptacle is found on a host or hub, and a series A plug is at the end of the cable that attaches to the host or hub.

Figure 11-6. Series ...