Why Incorporate USB Into the Macintosh Architecture?

The motivation behind the selection of USB for the Macintosh architecture is simple.

In addition to the obvious performance advantages, USB devices are hot pluggable and as such provide a true plug and play experience for computer users. USB devices can be plugged into and unplugged from the USB anytime without having to restart the system. The appropriate USB device drivers are dynamically loaded and unloaded as necessary by the Macintosh USB system software to support hot plugging and unplugging.

Better Device Expansion Model

The USB specification includes support for up to 127 simultaneously available devices on a single computer system. (One device ID is taken by the root hub.) To connect and use USB devices, it isn't necessary to open up the system and add additional expansion cards. Device expansion is accomplished with the addition of external USB multiport hubs. Hubs can also embedded in USB devices like keyboards and monitors, which provide device expansion in much the same way that the Apple Desktop Bus (ADB) is extended for the addition of a mouse through the keyboard or monitor. However, the USB implementation doesn't have the device expansion or speed limitations that ADB does.

Compact Connectors and Cables

USB devices utilize a compact 4-pin connector rather than the larger 8- to 25-pin connectors typically found on RS-232 and RS-422 serial devices. This results in smaller cables with less bulk. The compact USB connector provides two pins for power and two for data I/O. Power on the cable relieves hardware manufacturers of low-power USB devices from having to develop both a peripheral device and an external power supply, thereby reducing the cost of USB peripheral devices for manufacturers and consumers.

The cables for high-speed and low-speed devices differ in construction. High-speed USB device cables require shielding and two pairs of twisted-pair wires inside. One twisted pair provides power, nominally +5V (4.3 to 5.3 V at 100ma) for devices connected directly to the host, and ground. A powered hub can provide up to 500ma of +5V per port. (See USB Hub Devices for a description of the services a hub provides on the USB.) The other pair of wires is for data I/O signals. (Low-speed cables are untwisted and do not require shielding.)

High-speed cables are most common, and appear as patch cables to attach hubs to hubs, or attach high-speed devices to a hub. Low-speed cable length can be up to 3 meters, and high-speed cable length up to 5 meters. Both high-speed and low-speed cables can be used on the same system bus.

USB cables are directional, the upstream connector is mechanically different from the downstream connector. The upstream connector has a small nearly square shape with a stacked pinout and the downstream connector has rectangular shape with an in-line pinout. This prevents users from connecting cables in a way that would create a loopback connection at a hub.

Use of Standard Hardware

Devices that are designed in accordance with the USB standard should not require any modification to run on a Macintosh computer or other hardware platforms. The only changes that developers need be concerned with to support the Macintosh market are the changes involved in the development of Macintosh USB device drivers and applications.

Lower Cost Than Comparable Non-USB Peripherals

Low-power USB devices are less expensive than their serial or parallel interface counterparts, because of the elimination of the power supply and because the USB standard is also incorporated into PC systems developed around the PC '98 hardware architecture. Future versions of the PC `98 compliant operating systems will also include built-in driver support for a wide variety of USB devices. Together these factors mean that a larger customer base will form for USB peripheral devices, resulting in lower retail costs of USB devices for all personal computer users.