IEEE 1394 (FireWire)
The standard provides a guaranteed theoretical bandwidth of up to 50 MB/s and allows data transfer over distances of up to 4.5 m for IEEE 1394a at 400 Mb/s and 14 m at 200 Mb/s. Depending on the set-up and the quality of the cabling, longer distances can be achieved.
In approximately 2004 the next generation FireWire called IEEE 1394b was developed, which increased the data rate up to a guaranteed bandwidth of 100 MB/s (800 Mb/s) with tests showing that 83 MB/s is possible in real world conditions. While the future roadmap for FireWire includes proposed standards with speeds of 1600 and 3200 Mb/s, at the time of writing there is no evidence that the machine vision industry will adopt them with USB 3.0 becoming the interface of choice for highspeed host based interfacing. IEEE 1394b also adds three other cabling options. CAT5 twisted pair, delivering 100 Mb/s over a distance of 100 m; Plastic Optical Fibre (POF) giving 200 Mb/s over a distance of 100 m and Glass Optical Fibre (GOF) giving 800 Mb/s over a distance of 100 metres, with tests showing that 400 m lengths are possible. For vision use, GOF represents the only viable solution for long distances due to the limited data rates of the other cabling options.
A dedicated uncompressed IEEE 1394 video standard for industrial and scientific applications has been developed by the IIDC (Instrumentation & Industrial Digital Camera Working Group) called the DCAM standard. It offers true plug and play functionality, removing the need for camera definition files and time consuming set-ups. DCAM defines a standard register map for the camera, standard resolutions and standard frame rates. To provide flexibility, the standard also includes an arbitrary format called Format 7, which provides user registers that allow new features to be added that are not defined in the DCAM standard. As most cameras include a number of of non-standard "smart features" such as time stamping, delayed read-out and partial scan, DCAM has lost its plug and play advantage with camera manufacturers providing dedicated SDK's to control the specialist features of their cameras.
FireWire remains one of the best selling digital camera interfaces for slower speed cameras in terms of units shipped per year, however, for new applications its generally being superseded by USB3 Vision and GigE Vision.
FireWire topographical models
IEEE 1394 is a bus system that can have up to 63 nodes of which 16 can be cameras. The architecture provides two communication channels, one asynchronous channel for camera register control and an isochronous, (asynchronous data transfer over a synchronous link) which uses DMA channels to deliver data with minimum or no CPU overhead. The number of DMA channels on the host PC interface defines the number of cameras that can be active at the same time and is typically 4, although some cards can support up to 8 cameras.
Cabling and locking mechanisms
One concern that vision developers have had with IEEE 1394 is the fact that the standard does not define locking connectors or robotic grade cables. However, STEMMER IMAGING offers a range of cables that meet these needs. They are available with or without locking options and robot flexibility and are fully compatible with our range of cameras and interface cards. For a more in-depth discussion of the different connector types, please refer to the cabling section.
At the time of writing we started to see a few camera products being announced using Thunderbolt, Light Peak and PCIe interfaces. All of these interfaces provide the option for one of the existing camera standards to sit on top. These standards effectively extend the internal PC, but to a copper or fiber wired connection, allowing along frame grabbers to become external to the PC, or a ultra high-speed camera to connect directly to the PC.
Legacy parallel digital interfacing
The first generation of cameras that required a digital output in the 1980's and 90's used parallel digital; firstly RS422 and later LVDS. There are still some cameras that use this technology, but as there is no standard, every combination of camera and frame grabber uses a different connector, which makes cabling very complex. Because the speed of the interface is limited, many cameras use multiple channels or taps to increase data throughput and many of these digital cameras offer high dynamic range output that requires 10, 12, 14 or even 16 bits per pixel. LVDS and RS422 require two wires per signal, therefore, for a 16-bit camera with two output taps the required cable contains about 70 wires, including timing and control. Parallel digital interfacing has largely been replaced by CameraLink.
Legacy analogue interfacing
The simplest form of analogue interface uses a single 75 ohm coaxial cable that transmits the video and timing information using the same signal. For colour cameras, the colour information is integrated into the signal to create either PAL or NTSC signals. Trying to fit all this information down one wire naturally limits the image quality, and so for colour applications, the colour (or chrominance) information is often transmitted over a separate cable from the luminance (monochrome) signal. This system is known as the S-Video standard. To maintain the best possible colour image quality using analogue technology, each colour signal uses a separate channel, requiring 3 cables: one each for Red, Green and Blue. This interface is called RGB. Within the vision industry there is also an additional requirement to support dynamic exposure control and asynchronous triggering. To achieve these, the timing signals are usually separated from the video signal and this additional exposure control requires two or three extra cables. Although there are no official standards, the Hirose connector has become the de-facto standard for analogue vision cameras, making interfacing relatively easy.
In the digital era analogue cameras are generally not specified in new applications and with Sony's announcement of the closing of their CCD factory we expect this standard to die by 2025 at the latest.