CameraLink

As an absolute high-end interface with the highest possible data rate, the CameraLink standard is well established in machine vision and completely replaced the LVDS interface.

CameraLink (CL) is a standard which was created by the Automated Imaging Association (AIA). The members of AIA are many renowned camera, frame grabber and software manufacturers in industrial machine vision. CL is an advancement of the Channellink protocol and was created in 2000 in the specification 1.0 especially for secure image transmission of large data quantities. The current standard is version 2.0, which was released in November 2011.

Advantages of the CameraLink interface

• CameraLink provides the user with the highest data rate of up to 800 MB/s. High-resolution image signals of fast area scan or line scan cameras can be transmitted in this way. Almost every important supplier of high-end cameras for machine vision offers CameraLink cameras.
• The digital image transmission is free of loss and takes place via standardised signals, cables and plugs. Compared to the LVDS interface, now all cameras and frame grabbers of all manufacturers use the same plug and socket. The plug connections are screwed or clipped and therefore industry-suited.
• CameraLink as an industry standard reduces the costs for cameras, frame grabbers, and especially for cables due to this compatibility.
• In addition to data conductors for the image transfer, CameraLink interfaces integrate a two-core serial interface for the configuration and control of the camera as well as another four conductors for the transmission of trigger and synchronisation signals. This external triggering allows for a very low release delay and guarantees a nearly real-time capable control of the camera.
• In addition to the CameraLink cable, another connector for the voltage supply of the camera is required. The latest specification 1.2 of CameraLink also introduces a PowerOverCameraLink (PoCL) and a MiniCameraLink plug connector with PoCL in order that only one cable must be used. The first new cameras for MiniCameraLink were launched in 2008.

Characteristics of CameraLink

• CameraLink is a serial high-speed point-to-point connection. A maximum of two cameras can be connected to a computer system using one frame grabber. Further cameras require the use of additional frame grabber cards.
• The bit width per pixel can vary between 8 and 16 bits. The maximum pixel frequency of the data transfer is 85 MHz. If even more data must be transmitted, this can be done in several parallel data streams. They are called taps.

CameraLink is available in three different variations:

• Base (maximum 24 bits per cycle) – approx. 255 MB/s using 3 taps of 8 bits each (3 bytes * 85 MHz)
• Medium (maximum 48 bits per cycle) - approx. 510 MB/s
• Full (maximum 64 bits per cycle) - approx. 680 MB/s using 8 taps, with special tricks even more (800 MByte/s)

• CameraLink cameras are connected to a PC system by means of frame grabbers. These plug-in cards for the PCI bus or PCI Express bus can transfer the image data to the internal memory by using DMA channels with extremely low processor load. Therefore the image data transmission via CameraLink requires extremely little processor capacity of the system.
• The CameraLink specification defines a standard for cables, connections, signal format and serial communication interface. Yet, as opposed to FireWire and, in the optimal case, GigE vision, the communication between camera and computer is not defined by a special standard. For correct operation a suitable config file is required which tells the frame grabber and software how the images are transferred and via which serial commands the camera is controlled.
• Via the standardised cable connection using an MDR26 plug, not the total bandwidth of the medium or full specification can be realised. Instead of using different cables and plugs, communication is established via a second CameraLink cable and a second connection socket on camera and frame grabber (see above picture).
• Typical cable lengths realised using CameraLink are approximately 10 metres. When using particularly high-quality cables, frame grabbers and cameras, this specification can slightly be exceed in case of lower pixel frequency. MiniCL plugs and cables provide the user with a somewhat lower performance due to the miniaturisation.
• If even longer distances must be bridged, repeaters or, for particularly long distances of up to 500 metres, optic fibre couplers must be used.

Disadvantages of CameraLink

• A frame grabber is required for the operation, which only provides connection possibilities for two cameras. Multi-camera systems can thus not be realised using CL.
• CL cameras are typically more expensive, the use of a frame grabber card also incurs higher additional costs.
• The cable lengths of 7 to 15 m are not sufficient for certain applications. Optic fibre transmission systems are very expensive.