Vision meets Ethernet
Jon Titus, Contributing Technical Editor -- Test & Measurement World, 4/1/2006
The emergence of protocols such as EtherNet/IP for industrial equipment and LXI for instrument control reveals a trend for using Ethernet for more than business communications and Web surfing. So, it comes as no surprise that vision-equipment vendors also have hoisted the Ethernet banner: The new GigE Vision protocol will use standard Gigabit Ethernet hardware to link cameras and PCs.
The Automated Imaging Association, which oversees the GigE Vision standards committee, expects approval of this new protocol by the organization's May Vision Show East gathering in Boston. But users of machine-vision equipment may ask, "We already have RS-170, USB, FireWire, and Camera Link, so why add another vision standard? And what does GigE Vision offer equipment users and developers?"
 |
| A single Ethernet switch can route GigE Vision images from one camera to a host PC and from one camera to several PCs. Courtesy of Pleora Technologies. |
A great deal, it turns out:
- Ethernet implementations require little new computer equipment. Most PCs come with Ethernet ports, and vendors offer high-performance network switches at low cost. If you have only a few cameras, you can make direct camera-to-PC connections without going through a switch. Of course, applications will require new GigEVision-compatible cameras. And software will need new drivers and a new Ethernet stack. The current Microsoft Windows Ethernet stack would cause a communication bottleneck.
- The GigE Vision standard will rely on a discovery process to obtain XML-coded data from networked devices. This information will identify camera types, characteristics, and options, so networked devices can operate in a plug-and-play fashion. Although USB and FireWire devices offer a similar capability, they suffer from distance limits that keep cameras close to computers.
- Developers who have had experience with Ethernet in industrial environments will find they can add GigE Vision hardware to existing networks without much effort. In some cases, developers will be able to mix vision and nonvision equipment on a network, although bandwidth may suffer.
- Ethernet protocols include error-detection mechanisms that can automatically request a retransmission of a defective data packet. Granted, USB communications include error checking, too, but those communications cannot reach the bandwidth of a Gigabit Ethernet link. And few consider USB to be an industrial network.
- Ethernet communications can occur over longer distances than those permitted for USB, FireWire, or Camera Link interfaces. A run of Ethernet Cat-5 cable, for example, can easily reach 100 ft.
Some users of GigE Vision equipment will simply connect one or more cameras to a host PC. But system designers may establish complex networks that route images through switches and networks not dedicated to vision applications. People in this latter group should consult with GigE Vision equipment and software vendors about bandwidth and latency issues as well as real-time response times. Although GigE Vision will simplify vision systems, it won't eliminate every headache.
|
