Connecting Peripherals

Modern Macintosh computers use two buses to attach the majority of peripherals. Traditionally, USB has been used for slower devices, such as keyboards and mice, while FireWire has been used for high-speed devices, such as hard drives, digital camcorders, and the like. However, with the advent of a faster USB specification, the distinction between the two buses is blurred.

USB

Although developed as an industrywide standard, the USB specification didn't take off until it premiered on the Macintosh line with the August 1998 iMac. USB was originally intended for slower, less demanding peripherals such as input devices, but in a pinch it can connect disk drives and cameras to the Mac.

USB supports up to 127 devices on one bus, but since most current Macintosh computers have two USB buses, theoretically you could connect as many as 254 devices to one Mac. To connect that many devices, you would need USB hubs, which provide multiple USB ports. Some devices, such as the USB keyboard that comes with iMac and Power Mac models, have built-in hubs. For example, you can connect another USB device through the port on the keyboard.

USB can supply a small amount of power to devices. The mouse, for instance, is powered by USB. As long as the device does not need more than 500 milliamps (mA) at 5 volts, the USB can supply the power. Devices that need more power, such as printers and scanners, require an external source of power. If a USB device is connected to a Macintosh running Mac OS X, and that device requires more power than the Macintosh can provide through that USB port, Mac OS X displays a dialog informing the user of this, to avoid overloading the bus. Built-in Macintosh USB ports provide more power than USB ports found on Apple keyboards.

USB 1.1 transfers data at either 1.5 megabits per second, or Mbit/s (low speed), or 12 Mbit/s (full speed); USB 2.0—which made its debut on the Macintosh line with the June 2003 introduction of the Power Mac G5—transfers data at up to 480 Mbit/s (high speed), compared to 10 Kbit/s for the old Apple Desktop Bus and 230 Kbit/s for serial ports. USB can transfer at any of its three speeds at the same time. If, for example, a low-speed keyboard and a high-speed camera are both connected, each will transfer data at its own speed. Furthermore, you can plug a USB 1.1 device into a USB 2.0 port, but you won't see any speed improvement. Conversely, you can plug a USB 2.0 device into a USB 1.1 port, but its speed will be limited to 12 Mbit/s.

USB is “hot-pluggable,” meaning you do not have to shut down or restart to connect peripherals. The Macintosh recognizes that a device has been connected and looks for drivers to communicate with the device.

A USB driver is a piece of software that allows the operating system and applications to communicate with the USB device. Some USB drivers are known as class drivers and work with a variety of devices. Apple includes two class drivers: one for keyboards and mice, and one for hard drives. Other devices, such as printers, may require their own drivers, which are available from the hardware vendors.

FireWire

FireWire—also known as IEEE (Institute of Electrical and Electronics Engineers) 1394-1995 and Sony iLink—is Apple's homegrown high-speed counterpart to USB. Like USB, FireWire is a serial bus that allows you to connect multiple devices (specifically, up to 63). However, there are considerable differences between USB and FireWire.

In January 1999, FireWire 400 (IEEE 1394) made its debut on the Macintosh line with the Power Mac G3 (Blue and White). As you may presume from its name, FireWire 400 supports data transfer at speeds up to 400 Mbit/s. FireWire 800 (IEEE 1394b) debuted with the Power Mac G4 (FW 800) in January 2003, supporting speeds up to 800 Mbit/s. Since the two specifications call for different types of connectors, some Macintosh computers sport both types of ports. You can't mistakenly plug a FireWire 400 device into a FireWire 800 port without an adapter (FireWire 800 devices won't work in FireWire 400 ports under any circumstances). When you connect a FireWire device, it makes itself known with an ID number and tells the bus whether it will communicate at 100, 200, 400, or 800 Mbit/s.

FireWire 400 cables use two pairs of wires to transfer data. The 6-pin FireWire cables have a third pair, which is used for transmitting power. FireWire with a 4-pin connector, consisting of two pairs of data-only lines, connects only to devices that provide their own power, such as camcorders. The key difference between FireWire and the Sony implementation of IEEE 1394, called iLink, is that Sony's specification covers only the 4-pin connector. FireWire 800 cables have 9-pin connectors.

FireWire devices can be connected to one another in a daisy chain or free-form branching, which allows for peer-to-peer communications between devices without the need of a host computer. For example, two digital camcorders connected to a Macintosh via the FireWire ports can transfer video between themselves without going through the Mac—even if the Mac is shut down. This free-form branching scheme also allows several Macintosh computers to share one device, such as a printer or scanner.

Most FireWire devices come with two ports and can be daisy-chained together. That is, you can connect one port to your Macintosh and the other to another device. However, there are some daisy-chain rules to keep in mind:

• Arrange devices in the chain from fastest to slowest, starting with the Macintosh. For example, when connecting a printer and a hard drive, make sure the chain goes to the hard drive first and then to the printer. Slower devices in front of faster devices can cause a bottleneck in the data stream.

• FireWire 400 cables cannot be longer than 15 feet, although FireWire 800 supports special glass optical fiber up to 328 feet.

• You can have a total of 63 devices connected to a single FireWire port, provided you use FireWire hubs to create multiple paths, none of which can contain more than 16 devices.

Finally, FireWire also supplies more power than USB. FireWire 400 supplies up to 15 watts at 10 to 24 volts, and FireWire 800 provides up to 45 watts, with a maximum of 1.5 amps and 30 volts. If you're familiar with setting ID switches and termination on Small Computer System Interface (SCSI; pronounced “SKUH-zee”) chains, you'll be glad to know that FireWire devices have no ID switches or termination. They are also hot-pluggable.