Acquisition: CameraLink HS
After an extended search for the optimal technical solution by a body of leading frame grabber producers, the HS-Link technology from Teledyne DALSA provided the first "proof of concept" which, in the guise of a product, was to demonstrate that the proposed concept really worked. It was decided to use components from the high-speed network technology to implement bandwidths of up to 300 MB/s with a single link, or "lane" in CameraLink HS jargon, and up to 6 GB/s with up to 20 lanes.
A complete CameraLink HS core can be integrated in the FPGA making the integration of the technology cost-effective with a ready-to-use IP core available to developers.
Particular attention was given to the triggering capability of the interface by adapting the link layer protocol and the adoption of special key codes. An extremely low trigger jitter of only 3.2 ns, can be sent from the frame grabber to the camera, making CameraLink HS excellent for line scan camera applications where high trigger frequencies can occur that must be transmitted with little jitter.
To ensure data integrity, CameraLink HS includes a checksum and resend mechanism that is implemented in hardware on the FPGA and does not need additional memory capacity in the camera. This delivers a cost-effective solution with lower resources needed inside the camera, enabling cameras to be made smaller due to the lower hardware resources.
With CameraLink HS, as with CoaXPress, the cameras supply a GenICam GenApi XML device description. However, unlike CoaXPress, the device description may not be fully "visible" to the user because CameraLink HS does not require the use of GenICam GenTL. This means that the software interface of the camera is provided via the frame grabber software interface. The use of a GenTL driver for the frame grabber overcomes this potential issue.
With extremely high data rates a useful feature of CamerLink HS is data forwarding. Transmitters at the receiving end in the frame grabber can loop data with relatively little effort and expense. In this way, data from one source can be send to several frame grabbers either to enhance the computing capacity by bringing in several computers, or to establish a failsafe system.
The bandwidth of a single CameraLink HS link routed through one wire pair depends on the technology used. Thus, the maximum bandwidth of a link between frame grabber and camera is the result of the specified bit rate and the number of available data lanes. On the first configuration level with 4 links CameraLink HS uses a so-called passive CX4 cable with a cable length of up to 20 m. At the time of writing it is proving challenging to deliver extremely high flex versions of this cable. The standard also defines the use of fibre giving extended cable lengths of many hundreds of metres and there are also plans to offer a CameraLink HS interface with only one line to serve the low-end segment. With the challenge of making a high flex cable we expect further cable configurations to follow. The integration of the power supply source is a possibility, but not a reality in the early products currently available.
To document the compatibility and the features of the different configuration levels of a CameraLink HS product there is a CLHS capability designator for every product, which is composed as follows:
Capability designator: A-B, CDE|CDE, F
|Field||Long description||Short description|
|A||Number of connectors for video transmission||QuantityConnector|
|C||Maximum number of data lanes in the cable||#DataLanes|
|D||Protocol type||Link Protocol|
|E||Number of command channels||#CommandChannels|
|F||Bit transfer rate||Transfer Speed|
For example: "C2,7M1, S3" designates the following:
|Erstes Feld leer||If the first field is left blank, then a single connector isdesignated|
|»C2«||The connector type is a copper, SFF-8470 withthumbscrew retention|
|»7«||Supports up to seven lanes of data|
|»M«||Protocol is multi-camera, multi-lane|
|»1«||Supports a single command channel|
|»S3«||Bitrate is 3,125 Gb/s. (Can be left blank if 3,125 Gb/sis the connector's default rate)|
CameraLink HS resides under the management of the AIA, the Automated Imaging Association. Like CoaXPress, this standard was adopted by the G3 consortium of AIA, EMVA and JIIA, the worlds three machine vision trade associations covering all continents. For more information see http://www.machinevisiononline.org.