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Feature Articles

Camera Link 1.2: Power in Small Spaces

by Winn Hardin, Contributing Editor - AIA


Camera Link in the Box
Camera Link is a hardware-based solution
for exchanging data between the camera
and host image processor unit based on
National Semiconductor's Channel Link
Established: Camera Link Standard 2000
Signal: 3.3 V low voltage differential based
   on 11 shielded pairs w/ 4 drain conductors
Data format: Total of 28 bits = Video data,
   24 bits + Camera control 4 bits
Chipset speed: 40-85 MHz
Bandwidth: up to 680 MBps in Base/single
   cable mode @ 66 Mhz, or 850 MBps
   @ 85 Mhz.
Maximum cable length: Original standard
   up to 10 m

During the past year, the Camera Link standard committee, hosted by the Automated Imaging Association (AIA), has worked to expand and improve the standard to make it more applicable to a wider variety of imaging applications, as well as to give vendors the capability to differentiate their Camera Link products.

In late 2006 and early 2007, the Camera Link committee approved two separate annexes to the Camera Link standard. Although the Camera Link standard has traditionally defined cables by their physical characteristics such as gauge, shielding, insulation, finish and length; the new Annex D adds electrical descriptions to the standard, giving cable manufactures the opportunity to create unique cables optimized for varying combinations of length, diameter and flexibility. Annex D also defines a new miniature Camera Link connector based on Honda's HDR or 3M's SDR connector, allowing Camera Link to be integrated into the new generation of miniature and remote head cameras.

A new Annex E provides an optional feature to power and control the camera using a single cable similar to USB and FireWire. Adding a Power over Camera Link (PoCL) capability eliminates the need for a separate power cable while providing full backward compatibility for systems that do not use PoCL.

Connectors & Cables
The miniature connector option included in the Camera Link 1.2 standard (a finalized version of which should come out late this summer as version 2.0) is based on the HDR or SDR connector, which is roughly half the height and width of existing MDR Camera Link connectors, according to Steve Kinney, Product Manager at JAI and Co-Chair of the Camera Link committee. The smaller connectors are pin-for-pin compatible with their larger brethren. Many cables come in a similar format to miniature USB connectors, with a large MDR connector at one end for the frame grabber, and an SDR or HDR connector at the other end for the camera.

The advent of the miniature connector meant that the Camera Link committee also had to revisit the Camera Link cable specification. The committee found that the connector could accommodate 28 gauge wire, but only with some customization of the connector assembly. Manufacturers of the miniature Camera Link connector specified 30 gauge as the maximum recommended wire thickness. ‘‘When Camera Link was first introduced and the 10 meter signal transmission distance decided upon, there weren't many 85 MHz cameras,’‘ explains Jeff Fryman, Director of Standards Development at AIA. ‘‘With 30 gauge wire, it becomes a physics issue. The 85 MHz signal will not go 10 meters on 30 gauge wire without the use of some very exotic and expensive materials.’‘

In response, the Camera Link committee added a table to Annex D that shows transient rates and tolerances that must be adhered to by cables, regardless of gauge or length. ‘‘Camera Link cable manufacturers will now fill out a declaration for each product certified by AIA, the custodians of the standard and certified equipment vendor list, that shows how far a certain cable will operate at the maximum 85 MHz,’‘ says JAI's Kinney.

Although an initial view might be that the miniature Camera Link connector and smaller gauge wire would limit Camera Link's applicability by shortening maximum cable runs, Kinney says the new electrical definitions will actually have the opposite effect.

‘‘We define the resistance, skew, and other electrical specifications and allow the vision system designer to make the final decisions. For instance, the new standard says you need an inner and outer shield, but doesn't say what those materials are. Some users may be willing to pay for the use of exotic materials that actually allow longer transmission of Camera Link standards beyond the 10 meter limitation at 85 MHz,’‘ Kinney notes. ‘‘Cable manufacturers also should be better served. They can make a Camera Link cable close to a USB cable in size and diameter, but that cable might only go two feet. But there's a lot of machine vision equipment and other stuff that use short interconnects. Conversely, if you have a Camera Link cable for a robot arm and don't care about diameter, but need the cable to withstand 400,000 cycles around a mandrill, the Camera Link cable manufacturer can do that too. Before they were limited by what they could do during cable construction. Now they'll have a more involved declaration process, but otherwise, they can define their own maximum distance at 85 MHz, and can make everything from flexible to larger diameter cables that go longer than 10m.'

Power over Camera Link (PoCL)
During cable discussions, the Camera Link standard committee also saw an opportunity to address a long-standing concern from the broader imaging community about providing power over Camera Link connections and eliminating the need for a separate power conductor. The result was Annex E.

PoCLOne of the main concerns for PoCL was how to address backward compatibility and not expose non-powered Camera Link systems to potentially destructive currents. Luckily, the initial Camera Link standard included additional ground conductors – four to be exact. Version 1.2 now isolates two of the four grounding conductors to carry electrical power from a PoCL-enabled framegrabber to a PoCL-enabled camera. If you plug it into a legacy camera, there's not a short issue because the jacket shielding still protects the Camera Link components.

PoCL does place some additional component requirements on the framegrabber. The framegrabber needs a sensor and overcurrent register to measure impedance on the power pairs to sense whether to power the cable or not, if the camera isn't PoCL compatible.

Certified to Spec
The most important changes with Camera Link 1.2 (soon to be 2.0) are the need for additional attention to specifying cables and the new PoCL capability. ‘‘The big thing is to pay close attention to your need in selecting a cable, particularly if you're going for smaller connectors and cables sizes at high frequency,’‘ notes AIA's Fryman. ‘‘All Power over Camera Link compatible systems certified by the AIA will carry the PoCL logo and be listed on our website. PoCL certified cables will function perfectly in old systems if you have a non-PoCL application, but if you have a PoCL component, it will only work if you have PoCL cable to go with it. We put a lot of work into making sure all legacy equipment would be safeguarded.’‘

Interested parties can purchase the new Camera Link 1.2 standard on the Machine Vision Online Bookstore.







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