One camera standard or many?

Cameras based on Firewire (IEEE 1394), USB 2.0, and Gigabit Ethernet offer a convenient alternative to their Camera Link counterparts, because they connect to standard computer ports through a simple cable. Camera Link cameras, in contrast, require a separate frame-grabber board and a fairly large cable. Yet, engineers continue to debate the merits of using the Firewire, USB 2.0, and Gigabit Ethernet buses as serial communication channels between machine-vision cameras and computers.

Jerry Fife, a product manager at Sony, says three misconceptions hamper proper understanding of the IEEE 1394 bus. First, although the bus can run only 4.5 m in a single hop, using hubs can extend this distance to about 72 m. (The newer IEEE 1394b standard permits communications over a 100-m fiber-optic link.) Second, users misunderstand triggering. A 1394 camera doesn't have to wait for a signal coming down the bus. Most cameras can accept an external trigger, so if timing is critical, image acquisition (in a Sony camera) can begin about 4 µs after receipt of an external trigger. Third, people think few suppliers exist. But companies such as Basler and Meta-Control Technologies now offer 1394-based cameras.

A Firewire (IEEE 1394) camera can operate over a serial-communication channel. Manufacturers offer a variety of resolutions and frame rates. Courtesy of Sony.

Vieth notes that Gigabit Ethernet will operate faster than either IEEE 1394 or USB. One of the drawbacks of Gigabit Ethernet is the lack of software-interface standards. Vieth says the Automated Imaging Association (AIA) may look into software standards.

Some people won't rely on Ethernet in real-time applications because Ethernet operates with a "collision-sense multiple-access/collision detection" (CSMA/CD) protocol. To put it simply, increasing traffic on a network can delay transmissions. Recent adaptations of Ethernet for real-time voice transmissions, however, may help Ethernet overcome timing objections.

These days, most computers come with USB ports, so a USB-based camera seems natural. Yet, with only a few exceptions, USB cameras seem limited to consumer markets in which people snap still images. Silicon Imaging offers several cameras that provide USB 2.0 outputs, but the company notes as one of its top-10 requested features the plug-and-play Camera Link interface. The jury is still out on USB.

Although the digital camera standards get a lot of attention, most vision applications still use basic analog cameras and frame grabbers. Maybe the decision about what bus to use isn't that complicated after all.

Jon Titus, Contributing Technical Editor, jontitus@comcast.net