USB

Universal Serial Bus is an industry standard for connecting peripheral devices to the computer. With USB, you can connect a variety of devices, such as printers, scanners, external storage devices, keyboards, and joysticks. USB is built in to all Macintosh computers since the original iMac shipped in 1998. Many Apple computers have two USB ports. USB plugs, ports, and cables are often labeled with this standard USB symbol:

Mac OS X includes several USB kernel extensions, installed in /System/Library/Extensions, that are read into memory as Mac OS X is starting up. This provides access to USB keyboards and other devices very early in the startup process.

USB Connectors

USB devices use two types of plugs: Type A and Type B. The Type A plug is flat and connects to the host computer or an upstream hub. The Type B plug, which is somewhat house-shaped, connects downstream to the USB device. (Some devices, such as the mouse, don't have the Type B plug, because the USB cable is soldered into the device.) The connectors have four pins, two for power and two for data.

Macintosh USB system software version 1.3 supports all four data transfer types defined in the USB specification. USB devices can provide a remote wake-up function for the computer. The USB root hub in the computer is set to support remote wake-up whenever a device is attached to or disconnected from the bus.

USB Components

USB requires several components:

• Operating system support. Mac OS has supported USB since version 8.1.

• A host to control the USB system and manage connected USB devices. A USB chain can have only one host (typically, a controller chipset in the computer), which is why USB can't connect computers—hosts—to each other, even though its data transfer speeds can be comparable to Ethernet. The USB host typically provides two or four Type A ports connected to an internal hub.

• A hub, for connecting USB devices. Only hubs have Type A ports, so if you see a peripheral, such as a keyboard or display, with Type A ports for additional devices, it must have its own internal hub. This is good to know when troubleshooting hub and device connections. Freestanding hubs may be “bus-powered” by the USB or “self-powered” by an AC adapter. Hubs can also connect to other hubs, providing expansion to the 127-device limit. Most freestanding external hubs provide four or seven ports.

Rules for a USB Chain

Following are some considerations when you connect USB devices in a chain:

• There can be a maximum of 127 devices attached to one USB bus.

• There can be a maximum of six tiers, including the root hub inside the computer. This can be tricky to track sometimes, since hubs can be hidden inside the computer or on displays, keyboards, and so on.

• Bus-powered hubs can't connect to other bus-powered hubs, because the bus doesn't provide enough power. A keyboard with an internal hub is a good example of a bus-powered hub. Instead, to extend the chain of USB devices, you have to alternate self-powered and bus-powered hubs, or use all self-powered hubs.

• USB cables can be up to 16 feet long and typically come in approximately three-, six-, 10-, and 16-foot lengths.

Connecting Multiple USB Devices

To connect more than one USB device to a computer with a single USB port, use a USB hub. Hubs are devices that have multiple USB ports, as previously described. You connect USB devices to the hub, and then connect the hub to the computer. You don't need to install any software to use a USB hub.