Coaxial cables were in common use during the initial years of imaging for analogue cameras, providing a cost-efficient and robust solution with marginal signal loss over long cable lengths. In the course of the digital (r)evolution, these traditional cables are now also making inroads in both, CoaXPress digital transmission standard and also the HD-SDI broadcast standard. For this, a parallel digital signal which is generated by the camera is serialised and then send to the frame grabber through a single 75 ohm coaxial line in data packages at a correspondingly higher frequency and with electronic conditioning. Due to the fact that CoaXPress, unlike IEEE 1394 or Gigabit Ethernet, is not using standard mass market technologies, the CoaXPress specification comprises the electrical specification of the signals on the cable, in addition to the protocol definition. CoaXPress establishes a point-to-point link between the camera and the frame grabber, so that the complete bandwidth is always available to the image data of the camera.
As described, in the simplest case, CoaXPress uses a single coaxial line for forward and return channels, so that it is fairly easy to route the connection of the camera through a slip ring to provide a movable camera installation with little effort. Unlike in the case of GigE Vision or IEEE 1394, a frame grabber is needed which receives the data in the PC and sends control data to the camera. The drawback of the frame grabber is that an additional card must be installed in the PC; its advantage is that extra preprocessing can be integrated on it and higher bandwidth which was the original intention of CXP.
Coaxial cables, legacy from many older analogue installations, make conversion to digital technology with higher bandwidth a simple and low-cost effort. However, special care should be taken as not all old cables can be used for CoaXPress, please get in touch for detailed advice.
To ensure data integrity, CoaXPress uses a CRC checksum for the image data. Unlike, e.g. with CameraLink, bit errors can be detected during the transmission with this arrangement. However, to reduce the level of complexity at the receiving end and not having to install additional memory capacity in the camera, no resend mechanism, (by which corrupt packages can be resent to the frame grabber), is provided.
The camera is triggered either via an input on the camera or via the 20 Mb/s uplink from the frame grabber used for camera control. As a result of this, trigger signals have a fixed latency of 3.4 µs with a jitter of only 4 ns.
The connector in the one-cable solution is a commercial coaxial BNC or DIN 1.0/2.3 plug. More and more frame grabber manufactures are now using the DIN connector.
A single cable can transmit up to 6.25 Gb/s from the device to the frame grabber and 20 Mb/s of control data from the frame grabber to the device which - as far as image data is concerned - is equal to approximately 5 or 6 times the GigE bandwidth. However, some CoaXPress cameras and frame grabbers in the market can run at half speed 3.125 Gb/s.
The voltage supply to the device is provided through a CoaXPress link 24 V at a maximum of 13 watt, which is sufficient for many cameras, however, for more demanding power needs, the camera can be supplied with voltage in the conventional manner. The maximum cable length is 25 m at the full bandwidth of 6.25 Gb/s, or 65 m at half the bandwidth of 3.125 Gb/s (at time of writing).
If a single link is not sufficient, several cables can be combined to use several links in parallel to reach higher bandwidth. At the time of writing cameras and frame grabbers are available with 4 parallel links.
The software support of CoaXPress also strikes out in a new direction being the first standard to require the provision of a GenICam GenTL for the frame grabber. As with the other modern machine vision camera standards, CoaXPress must provide a GenICam GenApi XML file with descriptions of the features provided by the camera. This makes CoaXPress the first standard in industrial imaging that provides a uniform software interface for enumerating and controlling cameras.
CoaXPress was presented by a consortium of firms at VISION 2010 for the first time. Today this standard is hosted by JIIA, the Japan Industrial Imaging Association, a member of the G3 consortium of AIA (Automated Imaging Association), EMVA (European Machine Vision Association) and JIIA. The specification of CoaXPress is freely available in the web. For further information, see http://www.jiia.org or www.coaxpress.com.