Camera Link standard—a necessity

During the past ten years in the machine-vision and image-processing industry, interfacing technology has steadily evolved.

GUNNAR JONSON is director of product marketing, JAI Camera Solutions, Copenhagen, Denmark; www.jai.com.

During the past ten years in the machine-vision and image-processing industry, interfacing technology has steadily evolved. Starting with common video-broadcast technology with its standards, such as interlaced image readout and analog video transmission through coaxial cables, vision technology has moved to special image sensors with high-speed transmission systems and distinctive image-acquisition platforms.

As digital interfaces started to emerge, they were initially based on TTL-level signaling, with all its inherent limitations in cable length and noise immunity. Soon thereafter, manufacturers started migrating to stable RS-422 differential signaling, which greatly improved camera-to-frame-grabber interfacing. For many years, RS-422-based interfaces dominated digital interfaces and were considered sufficient for most industrial vision applications.

Certain, more exotic applications requiring longer transmission distances turned to special solutions based on Cypress Semiconductor (San Jose, CA; www.cypress.com) HOTlink or similar chip sets. These platforms, however, remained in the proprietary domain and so did not gain wide acceptance. As the need for higher speed emerged, the industry turned to a new generation of digital-signal transmission—low voltage differential signaling (LVDS), or EIA-644. Still specified in a parallel format, LVDS satisfied the need for higher speed, but did not reduce the complexity of the cabling and interfacing. It has achieved moderate implementation.

Around this time, cameras with multiple outputs started to enter the market, but wide acceptance of such cameras was slowed by the lack of suitable image-capture boards and practical cabling. At a typical user's site, racks upon racks of image-acquisition boards had to be connected to accommodate several 8-tap cameras via a cabling system that resembled a nest of vipers. Obviously, such system architectures were marginally successful. The high cost factor was also inhibiting, and the low-level reliability of such a complex system contributed to the limited proliferation of this technology.

Many technologists exclaim that interface cabling is the major stumbling block to market growth in the machine-vision industry. In contrast, other industry technologies have spurred vision expansion. Cameras with higher speeds and multiple outputs have appeared. Progressive-scan systems are gradually replacing broadcast-based interlaced scanning systems. On the image-acquisition side both the speed and the width of the interface bus have greatly increased. But, at the same time, little has changed in the physical interfacing and cabling technologies over the past decade. As a result, the Camera Link standard for interfacing cameras to frame grabbers has enjoyed wide acceptance in the imaging industry within a short time.

Over the years, camera manufacturers have continuously discussed interfacing issues with OEM customers. Many different interfacing schemes were evaluated, mostly based on parallel-to-serial conversion and transmission over optical fibers, twisted-wire pairs, or coaxial cables. Some OEMs have designed clever transmission systems. However, by including their own proprietary interfaces, these OEMs confronted buyer resistance.

Practical standard

From the basics of Channel Link technology came Camera Link. Overwhelming support for Camera Link toward standardization evolved when the Automated Imaging Association (AIA; Ann Arbor, Mil www.machinevisiononline.org) accepted the responsibility of promoting this standard (see p. S3). This backing has enhanced the acceptability of Camera Link interfacing technology in the public domain and has turned it into a national and international standard.

Camera Link overcomes many drawbacks experienced in past methods. Even with multitap cameras, Camera Link interfacing is accomplished through a single 26-pin connector; the standard handles up to 8 taps with a second connector. In terms of speed, the Camera Link standard can comfortably handle up to a 66-MHz pixel rate, with the ability to expand to 85 MHz.

When the Camera Link is presented to customers, they frequently ask how this standard is positioned in comparison with IEEE 1394, or FireWire. Many customers see these two interfaces as competing standards. Naturally, there is some overlap between the two standards, but there are clear differences in their market focus. Camera Link is designed to serve the need for high-end cameras, where very-high-speed, high-pixel-count, multiple-tap architectures are required. As the automated imaging market continues to develop, more complex applications will arise, which calls for more complex camera and image-acquisition technology. Having Camera Link as a standard on which to base vision and imaging platforms that can meet complex application needs will help accelerate their market introduction. ..

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