Camera Link Cables For Machine Vision – Choose Correctly
Camera link has been a popular technology for transmitting high speed image data from industrial vision cameras for over a decade. The specifications and standard designations are maintained and administered by the Automated Imaging Association located in Ann Arbor Michigan, and is the global machine vision industry's trade group.
To its credit, the management of the technology has been so successful, independent manufactures are able to design and produce cameras, data acquisition devices, and interconnect cables and components with full confidence that they will integrate seamless with all other devices manufactured under these standards.
As Camera Link technology matured, changes in the standard were required to accommodate the demands of the machine vision industry for higher data throughputs and increasingly compact, lighter weight, components. The latter particularly critical to the robotics industry. The changes have resulted in a somewhat bewildering array of terms used to describe various interface configurations and cable or connector styles. The purpose of this article is to provide the end user a quick synopsis of these terms to aid in proper component selection for their projects.
Camera Link Configurations
The basic configuration is cleverly termed "Base" configuration and carries signals over a single connector / cable. The cable type was developed by 3M, and is referred to as a MDR ("Mini D Ribbon") and uses a 26-pin male plug connector on each end and is optimized for Low Voltage Digital Signals (LVDS). The cable carries five (5) LVDS conductor pairs transmitting twenty four (24) bits of serialized video data and four (4) timing signals. The connector also carries four (4) LVDS discrete control signals and two (2) LVDS asynchronous serial communication channels for communicating with the camera. The maximum video throughput is 2.04 Gbits / sec.
The "Medium" configuration doubles the video rate by adding an additional 24 bits of video data over a second cable, increasing maximum throughput to 4.08 Gbits / sec.
The "Full" configuration adds another 16-bits of video data, creating a 64-bit wide video path capable of carrying 5.44Gbit / sec.
The Camera Link cables are designed with 26 conductors, regardless of the configuration. The Base configuration requires just one cable, while both the Medium and Full configurations require two cables for the expanded data path.
Camera Link Connectors
The connector styles for all configurations are the same, each is a 26 pin connector. The original connector, developed by 3M, was termed a MDR connector. As industry demand for smaller and lighter components grew, 3M created the "Shrunk D Ribbon" connector design, which was termed a SDR connector. This design significantly reduced circuit board space and camera interface real estate and reduced weight.
More recently, Honda Connectors Ltd has introduced their own version of a high density, miniature Camera Link connector which they termed a HDR connector. Though this has caused some confusion for buyers tasked with purchasing cable and connector components, the HDR from Honda, and SDR from 3M Corporation are 100% compatible and interchangeable.
Extending the AIA Standard
Some camera and data acquisition hardware manufacturers have extended the bandwidth of the Camera Link interface beyond the limits imposed by the AIA specifications. These formats extend the width of the "Full" configuration by reassigning redundant timing signals in such a way as to produce a data path width of up to 80-bits, further increasing the video bandwidth However, such "nonstandard" configurations may lead to incompatibility issues with components manufactured within the imposed standards.
Although there appears to be a bewildering array of configurations and connector styles, the majority of cameras using the camera link interface support the simple Base Camera Link configuration with standard MDR connectors. Typically Medium or Full configurations are only required for very high resolution, high frame rate, applications. It is the limitations in architecture and costs of the imaging device that typically restrict performance, and not the data interface itself.
You will rarely encounter a camera device with an excess of 2.0 megapixels operating above 30 frames / sec and a 16 bit data output. Such a device requires a data rate of just under 1.0 Gbit / sec, easily managed by the Base Camera Link configuration limit of 2.04 Gbit / sec.