With the announcement of the iMac we started hearing a totally new abbreviation in the Macintosh world - USB. Gone were our familiar ADB ports and modem ports and printer ports, not to mention SCSI. Why is Apple moving to USB? What was wrong with serial ports and what's good about USB ports?
What Is USB? USB stands for "Universal Serial Bus" and is a special kind of serial port that's growing in popularity in the Windows world. Most late-model PCs have at least one USB port, and Windows 98 introduces plug-and-play support for devices attached to the USB port. (Bill Gates's widely reported COMDEX crash occurred as he was connecting a USB scanner to a PC running Windows 98. But that's no worse than how Windows plug-and-play often works.)
USB is intended to replace all the various types of low-to-medium-speed data ports hanging off the back of a PC (although most current PCs with USB also have at least some of the old-style ports). This includes not only the serial ports but also keyboard ports, mouse ports (which, on a PC, are basically another serial port), and parallel ports. Keep in mind that on a PC, the parallel port is used not just for printers but also for other devices such as Zip drives, tape drives, scanners, and even some modems.
Given that USB is intended to replace the parallel port as well as lower-speed serial ports, and to do it all at once, it's pretty speedy. USB devices can talk to the computer at two speeds: 1.5 Mbps or 12 Mbps (that's millions of bits per second, and remember, there are 8 bits per byte if you want to translate to bytes per second). In comparison, the Mac's serial ports max out at 230.4 Kbps, and ADB at just over 1 Kbps. Devices like keyboards and mice will use the slower speed; devices like Zip drives, printers, and scanners will use the faster speed. Both speeds can be connected to a single USB bus.
Ah, the bus. The term conjures images of the Apple Desktop Bus (ADB), Apple's standard way of connecting input devices to the computer, with its easy daisy-chaining. USB doesn't support daisy-chaining in the same way. Each USB port can host one and only one peripheral. However, that peripheral can be a device called a hub, which provides additional USB ports for more devices. Up to 127 devices can be connected to a single USB port using a collection of hubs. (Theoretically, a device could incorporate a "single-port hub" for daisy-chaining, but this isn't likely in the price-conscious PC market.)
Like ADB devices, USB devices can draw power directly from the bus, within limits. If you have many power-hungry USB devices, you'll want a powered hub to provide current for them. (The serious power hogs - things with moving parts and motors, like printers and scanners - will have their own power supplies and won't rely on the USB for power.)
USB Devices -- What kinds of devices will be available? Input devices, obviously: keyboards, mice, graphic tablets, joysticks, and so on. Anything that typically hangs off a PC parallel port is also fair game, which means we'll probably see USB Zip drives and other relatively slow mass storage devices (in fact, Imation and Panasonic have already announced a USB version of their LS-120 SuperDisk drive, which reads both 120 MB and 1.44 MB disks). Other USB candidates include tape drives, scanners, digital still cameras, modems, and printers. Newer Technology has also announced plans to create a USB-based floppy drive, specifically for the iMac. Many cable modems and ADSL adapters operate within the bandwidth of USB, so we might see those kinds of devices as well. The 12 Mbps variant of USB is faster than standard 10 Mbps Ethernet.
Although there will undoubtedly be USB Zip drives (or similar cartridge-type drives like the LS-120), don't expect decent performance from hard disks attached to the USB. Though a USB Zip drive will probably be faster than the PC parallel port version, and more than adequate for the kind of exercise a consumer is likely to give such a peripheral, SCSI is still faster. Even the slowest version of SCSI has a raw throughput of 5 MB per second, more than 3 times faster than USB, and the newer Ultra/Wide SCSI III can reach 40 MB per second. For hard disks and digital video cameras, you'll still want SCSI, or the ultra-high-speed serial port dubbed FireWire, which is yet another topic.
USB and the Mac -- How do Mac users benefit from USB? The obvious answer is that we can tap into the competitive jungle that is the PC marketplace. Before long, you'll be able to buy $15 keyboards just like your PC-using friends. (Of course, they'll be PC keyboards, but they'll work on your Mac.) Each USB device identifies itself through a generic "type" ("I'm a keyboard," "I'm a mouse," "I'm a Reality Distortion Field generator"), and a USB-compatible Mac will have a USB Manager with built-in drivers that let it talk to many devices in at least a minimal way. You'll need Mac-specific drivers to take full advantage of many peripherals, but it's a lot cheaper for manufacturers to create an extra piece of software than to make both a parallel port and a SCSI version of a removable-media disk drive for different markets. PC Cards work in much the same way now - the standard PC Card modem drivers work with almost any PC Card modem, but more specialized PC Cards require custom drivers. A few manufacturers have already announced Mac support for their USB peripherals. If the iMac takes off as retailers expect it to, many more manufacturers should follow suit.
USB has faced an uphill battle in the Windows world because of drivers. One of the primary reasons for the success of Windows over the years is that Microsoft includes a vast collection of drivers for different hardware devices with Windows itself, reducing installation difficulty and conflicts. However, since USB came out after Windows 95, drivers have all been provided by the individual USB peripheral developers, resulting in chaos. The just-released Windows 98 includes better USB support, so there's hope that the field will settle down. Apple's strategy of including drivers for common types of USB devices may make USB far more coherent on the Mac.
The iMac's keyboard, by the way, has a built-in two-port hub, so you can attach one additional device besides the mouse. The iMac itself has two independent USB ports (each with its own 12 Mbps bandwidth), which means that the stock iMac supports two additional USB peripherals (along with a mouse and keyboard), one connected directly to the computer and the second connected to the keyboard. If you need more USB ports, 4-port hubs run about $100 right now, but some observers expect them to fall to the $50 range as USB catches on, much as happened with Ethernet hubs.
Along with Newer Technology's announced plans to create a USB-to-serial converter, the rumor mill is hinting that at least one manufacturer will introduce a USB peripheral that will provide "old-style" Mac serial, ADB, and (really slow) SCSI ports, so users who move to an iMac from an older Mac can take at least their old printers and modems with them and hook up their old hard disks long enough to copy all their data over. An iMac with such an adapter and an ADB credit-card reader and barcode scanner would make a groovy-looking point-of-sale terminal (at least until there are Mac-compatible USB versions of these peripherals). Though it's never a good idea to put faith in rumors, this seems like an obvious product, if it can be produced at a reasonable price. Don't expect total software compatibility, though, as some software products unreasonably assume that no characteristics of serial ports ever change.
You can find out more about USB from a Web site operated by a USB industry consortium, and see what kinds of peripherals are available by visiting USB Stuff, a retailer of USB peripherals. Finally, MacInTouch has collected a variety of bits of information about USB contributed by readers.
[This article is reprinted and updated with permission from MWJ, the Weekly Journal for Serious Macintosh Users. If you can't get enough insightful Macintosh news, sign up for a free, no-obligation, two-issue trial subscription to MWJ, or download some of the free sample articles. For more information, see the MWJ Web site.]