USB: Adequate at the Low End

With the iMac, Apple is adopting the Universal Serial Bus (USB), replacing the SCSI, serial, and ADB ports Mac users have had for over a decade.

Certified USB 1.1 logoAt first glance, USB has a lot going for it. (Visit for more details.)

  • plug and play simplicity, even with power on
  • 12 Mbps bandwidth (faster than regular 10Base-T ethernet, which is 10 Mbps)
  • supports up to 127 devices
  • works with hubs or daisy chained devices
  • provides power to peripherals, up to 500 mA

And the list of USB products is extensive: keyboards, mice, joysticks, digital cameras, scanners, telephones, video cameras, speakers, displays, printers, and soon even disk drives.

Many Mac users already know USB is slow compared with SCSI – 12 Mbps is less than one-third the bandwidth of SCSI-1, and today’s Macs use SCSI-2 and SCSI-3, which are even faster protocols.

Potential Problems with USB

USB Users May Face Bandwidth Problems” in InfoWorld (8 June 1998, p. 14) notes some potential problems with USB.

For instance, every USB device has to share the same bandwidth. What happens when your USB speakers and USB video camera both want the bulk of your bandwidth?

That depends on which was plugged in first!

According to the article, you’ll either get good video and poor sound or poor video and good sound. (This is a good reason not to use USB for speakers!) USB is plug and play, but you’ll need to know the correct order for plugging things in.

Also, the USB specification calls for ports to provide power, but the amount varies. A passive hub, as in the iMac keyboard, need only supply 100 milliamps (Ma) to each connected device, while a powered hub (this included the iMac’s ports) must supply up to 500 milliamps.

If a device needs more power than is available, you won’t know it until you discover that the device hasn’t mounted. And at least on the Windows side, the OS won’t tell you when a device is connected but unable to mount. (Let’s hope Apple gets that right!)

What’s the Alternative to USB?

We can hope that Apple will offer FireWire as an option on the iMac and as a standard feature on future Power Books and Power Macs. A plug and play protocol for the power user, benefits of FireWire over USB include:

  • 8 to 32 times faster (100-400 Mbps)
  • provides up to 60W of power to peripherals
  • can connect between computers as well as to devices
  • faster than SCSI-3

While USB will be fine for modems, mice, keyboards, and joysticks, serious users will want the greater bandwidth of FireWire for external drives, live video, scanners, and other high throughput tasks.

Real World Throughput

In a column covering Macworld Expo [no longer online], Henry Norr notes a 0.7 MBps (megabytes per second) sustained throughput for the USB SparQ drive. This translates to 5.6 Mbps (megabits per second) – about half the potential bandwidth of USB. It is also comparable to the maximum real world throughput over 10Base-T ethernet.

Put in perspective, the best network backup speeds I see at work over a switched 10Base-T network with Retrospect and a DDS-2 tape drive is 42.8 MB/min. Assuming SparQ performance is typical for USB drives, 0.7 MB/sec. equals 42 MB/min. That’s not blazingly fast, but neither is it terribly slow.

Backing up 1 GB of data at this speed would take about 25 minutes, and perhaps a bit less if the backup software compresses data before writing it.

This means USB is not just viable for squirting small files to a floppy, Zip, SyQuest SparQ, or SuperDisk [the 120 MB floppy disk, not to be confused with Apple’s DVD-burning SuperDrive], but also acceptable for backup. SparQ media, at three 1 GB cartridges for $100, is also reasonably sized and priced for backup, making SparQ a very real alternative to the more expensive digital tape drives.


Apple has generated some real excitement for USB, even though it was invented on the Wintel side of the road. Apple is leveraging the expertise of those who have developed USB devices for the Windows market with the fact that USB has not even come close to replacing serial and parallel devices in the Wintel world.

With an expected 400,000 iMacs sold by the end of September, Apple may create the largest single market for USB devices – especially since iMac owners won’t have options such as ADB, serial, SCSI, or parallel ports.

Further Reading

Keywords: #usb

Short link:

searchword: imacusb