Advanced Imaging

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Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

GigE Vision Comes of Age

Advantages like high bandwidth, cable length, widespread use and network ease make it a major interface
An illustration of two cameras connected to a PC.
© DALSA
An illustration of two cameras connected to a PC. The first camera is Camera Link and has a short cable (10 meter maximum) while the other camera is connected by GigE (100 meter cable). Cable length is one of the strongest benefits of GigE Vision and a main reason why it can be used to replace analog cameras. Click here for larger image.
This is an illustration of a functional computer setup with two NIC ports, but no DHCP server
© DALSA
This is an illustration of a functional computer setup with two NIC ports, but no DHCP server. The first uses the default LLA mode for IP addresses, while the second NIC and the Genie cameras connected to it are configured with a persistent IP. An application on the computer can control each Genie camera, on each subnet, without conflict. Click here for larger image.
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By Eric Carey, DALSA

In the two and a half years since its introduction, GigE Vision has more than lived up to its promises. Not only has it seen widespread acceptance as a standard interface, it has increased the range of vision applications that digital cameras can serve. Enhancements to the standard due out this year, along with other refinements in the works, will take GigE Vision well along the way to becoming the major industry interface for cameras operating to 100 Mbytes/second.

When digital cameras using low-voltage differential signaling (LVDS) were first introduced to the machine vision industry, they created an interfacing challenge. While analog cameras offered standardized output signals, these digital camera signals varied substantially. This variation made it difficult for developers to utilize a camera from one vendor with a frame grabber from another vendor, impeding market acceptance of digital vision systems.

Early efforts to resolve this problem resulted in the creation of the Camera Link interface standard. Camera Link provides a standardized, high-bandwidth LVDS link for connecting a digital camera to a vision system, using a 26-wire connection between camera and frame grabber that handles data rates to 255 Mbytes/second. Using two connectors and dual cables allows data rates to 680 Mbytes/second. One result of the 26-wire connection, however, is that the cable required is both bulky and expensive, with a price as high as $250. The differential signaling scheme also suffers from a range limit of 10 meters, placing significant restrictions on system installation.

While Camera Link provides a standardized physical interface and image data, it does not specify the protocol for sending control information to the camera. Like the serial port on a PC, signals could be coded as binary data or ASCII characters, use checksums or not, and could even employ any number of different error correction coding schemes and still conform to the standard. As a result of this ambiguity in the standard, vendors developed differing formats for their control signaling, complicating developer efforts to mix-and-match cameras and frame grabbers in system design.

Embracing Ethernet

GigE Vision addressed these limitations by adopting Gigabit Ethernet as the camera link, allowing data rates to about 100 Mbytes/second. This interface provides enough bandwidth for all but the highest-speed cameras yet requires only a simple Category 6 unshielded twisted pair cable to carry the signals. Further, the cable can be as long as 100 meters. Gigabit Ethernet also has the advantage of being widely used across multiple industries, making the interface electronics and driver software both inexpensive and widely available.

GigE Vision brings other advantages to development of machine vision systems. Personal computers typically have Ethernet interfaces built in, so the GigE Vision interface allows developers to create a completed machine vision system using only a camera and a standard PC. The use of networks can extend the connection between the camera and the vision system to virtually any distance and allows multiple cameras to connect to the system simultaneously.

The interface also provides flexibility in the type of information it can carry. Camera Link was designed to provide image data at a high rate, and little else. Its relatively fixed data format made it difficult for the camera to send anything other than some type of image to the vision system. GigE Vision, however, provides data in software-defined packets that can contain virtually any type of information. This makes it easier for vendors to develop and customers to use cameras that offer built-in processing and provide the vision system with more than just an image.

These many advantages have fueled strong growth in the GigE Vision camera market since the specification was released in May 2006, with a large number of developers embracing the interface in the designs they have brought to the field. The interface has also created new opportunities for the application of digital cameras. The longer cable has allowed digital to replace analog in applications such as security and traffic surveillance. The flexible data structure has also allowed creation of intelligent cameras, such as an inspection system camera that automatically makes key measurements and sends the results but can also send an image to an operator if a part fails to meet specifications.

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