Camera Link
At a glance
Camera Link was released in 2000 as the first digital machine vision interface. Camera Link is a point-to-point semi-parallel/semi-serial data interface using multiple Low-Voltage Differential Signaling (LVDS) pairs and one or two 26-pin Miniature Delta Ribbon (MDR-26) or SDR-26 connectors. Different Camera Link Configurations are available, they range from Camera Link Base, with one connector / cable providing a bandwidth of 2 Gbps up to Camera Link 80-bit (Deca), with two connectors / cables providing a bandwidth of 6.8 Gbps (850 Mbytes/s). In addition to image transfer, four camera control signals provide direct, no latency control of the camera.
The Camera Link standard is hosted by A3 (Association for Advancing Automation). Although one of the oldest machine vision standards, Camera Link is still widely used, with hundreds of products such as cameras, frame grabbers, cables, extenders, and accessories available.
Technical summary
- Simple pixel data serialization, non-packet-based protocol
- Bidirectional serial communication to control the camera
- Power over Camera Link (PoCL)
- The camera clock frequency (and the frequency of the data transmission) starts from 40 MHz for older/cheaper cameras and frame grabbers and can reach 85 MHz for the latest devices
- Cable length in the range of 7 to 15 meters depending on the camera clock frequency
How does it work?
When it was launched, Camera Link used one, two or three Channel-Link transceiver chips from National Semiconductor. Each Channel-Link transceiver chip transfers 28 bits of data. The data are serialized 7 to 1, and the resulting four data streams plus a dedicated clock are driven over five LVDS pairs. The receiver accepts the four LVDS data streams and LVDS clock, and then drives the 28 bits and a clock to the receiving device.
Camera Link Base uses these 28 bits to carry up to 24 bits of pixel data and 3 video synchronization signals, leaving one spare bit. The video synchronization signals are Data Valid (DVAL), Frame Valid (FVAL), and Line Valid (LVAL). Camera Link Medium and Full respectively use one or two additional Channel-Link transceiver chips, for a total of up to 80 bits of data.
The camera clock frequency ranges between 40 and 85 MHz, the rate of the serialized data on the cable being seven times higher.
The Camera Link connector also carries four discrete LVDS control signals (CC1 to CC4) and two LVDS asynchronous serial communication lines for communicating with the camera.
Channel-Link is a general-purpose data pipe with no overhead for protocol or encoding. Hence, a Camera Link interface is relatively easy to implement. Contrary to CoaXPress, Camera Link HS, GigE Vision and USB3 Vision, the Camera Link data is simply serialized, it is not packetized, it is not encoded and there is thus no provision for error detection or error correction. Today’s Camera Link interfaces are usually directly implemented in FPGAs.
Latest features and future developments
More than twenty years after its launch, Camera Link remains a popular interface with hundreds of products such as cameras, frame grabbers, cables, extenders, and accessories available. In particular, it is still the interface of choice for line-scan cameras. Its main limitations are the bandwidth and cable length. Camera Link HS, an entirely new standard also hosted by A3, is not backward compatible with Camera Link and cannot be seen as a direct successor. In many machine vision applications, Camera Link has now been superseded by CoaXPress, which provides significantly more bandwidth, supports longer cables, and delivers the same level of robustness.