Q+A CENTER

Super Duper

Released on the heels of S3200, USB 3.0, also dubbed SuperSpeed USB, not only provides a 10X boost over USB 2.0, it reduces hardware requirements. "With USB 3.0, we have so much bandwidth that the designer can reduce the amount of buffering requirements on a device," says Rahman Ismail, USB-IF compliance chair. "SuperSpeed USB gives you not just the ability to increase traffic to your host, it also allows you to simplify your device and make it less expensive." The new release also offers improved power transmission capabilities compared to USB 2.0. A USB 3.0 device can draw up to 900 mA, up from 500 mA for USB 2.0.

All this performance requires not just modification to the chips but to the link, as well. The standard USB 2.0 cable contained four conductors: D+, D-, power, and ground. The USB 3.0 cable includes those conductors, plus two additional dedicated transmission lines, two dedicated receive lines, and an additional ground line for a total of nine. The upside is that the change allows full duplex, simultaneous transmission, although only in the case of a USB 3.0-enabled device, host, and cable. The downside is that each cable contains more than twice as many conductors—and points of failure—than did the previous release. In industrial applications, cabling is the single biggest point of failure, so integrators and machine builders will need to pay attention to cable-harness design and cable management with USB 3.0.

SuperSpeed USB provides impressive speed, though it is important to pay attention to definitions. The signaling rate is 5 Gb/second, but like FireWire, USB uses 8b/10b encoding; thus, the raw data transfer rate is actually 4 Gb/second. That's still a 25 percent increase over S3200, which begs the question of just how much speed is enough. Can vision applications actually use 4 Gb/second of bandwidth, or even 3.2 Gb/second?

The answer, surprisingly, is yes. "It's a combination of high pixel counts and high frame rates," says Scholles. "Also, more and more people are using color cameras or high-dynamic-range cameras where you have more than eight bits per pixel. If you multiply all that together, you can easily exceed the bandwidth of S800 FireWire."

"Maybe each individual camera doesn't need S3200 but if you want to combine them all on the same bus, you need high-speed operation," Mourn agrees. "Let's say I send a 2,000-byte packet every isochronous cycle on 1394. It's going to take a quarter of the time to send that packet at S3200 than at S800, so that means I could add four more cameras on the same bus."

In the end, it comes down to the application. What are your imaging requirements in terms of speed and resolution? How many cameras do you need and what type of network topology? How complex do you want the hardware to be? And most important of all, how much bandwidth does your application require?

There is no right or wrong standard. As in all engineering, the best standard to use is the one that performs most effectively in the task at hand.

Kristin Lewotsky is a freelance technology writer based in Amherst, N.H.