The standards maze
It seems impossible that the GigE Vision interface standard was only officially launched at last year’s Vision Show, so quick has its uptake been. With so many companies adopting the standard, it would be easy to believe that GigE, and the associated GenICam, has swamped the market. Dig a bit deeper, however, and you discover that FireWire and Camera Link themselves are developing, with soon-to-be-launched upgrades.
GigE Vision and GenICam
To summarise, GigE Vision provides a common standard for high-speed (1000 Mbps) image transfer using the Gigabit Ethernet communication protocol. The interface provides perfect data transfer over 100m using low-cost cables, and even larger distances are possible with a slight deterioration in transfer quality.
Clearly, the low cost, long distances and high speed have made this ideal for industrial and machine vision applications, an area where the standard was very quickly adopted. ‘More than half of our past 50 cameras were GigE,’ said Dr Friedrich Dierks, head of software development at Basler Vision Technologies. ‘However, there’s no reason why we should be restricted to machine vision. Soon other people will notice – it will take about one or two years to spread around.’
Another advantage of GigE Vision standard was the associated GenICam software interface, providing cameras with a plug and play capability. Each camera’s features are described using an XML file defined by the standard, allowing hardware-independent software to interpret the data. Properties of the image acquisition, such as the gain and area of interest, can be configured solely through the software.
The software interface has proved particularly popular with developers of software libraries, who no longer need to customise their products to each interface or manufacturer. For the user, it allows camera models in systems to be easily switched without the need for extensive programming. Possibly the biggest advantage is that it hides the interface: GigE, Camera Link, and FireWire can all communicate, allowing far greater flexibility for the most suitable interface to be used.
Ever since the launch of GigE there has been talk of upgrading the standard to 10GigE to allow even faster data transfer, at rates greater than those possible with Camera Link, currently the fastest interface. Dierks, however, believes this may pose a problem when the image is being processed. Currently, GigE sends each packet of information to the software to process; with greater speeds, this may overload the CPU. Rumour has it that a new standard is being developed that could solve this problem by offloading the data, and only informing the CPU when a whole image has been formed, allowing data rates of 2Gb/s or higher.
IEEE 1394 (Fire Wire)
‘GigE is a young technology,’ says Arnaud Destruels, the European product manager of machine vision at Sony. ‘To be sure it’s good enough for customers, it needs to be at least five years old. Maybe we’ll see its limitations soon.’
Surely this is a criticism no one could make of the IEEE 1394 interface, or FireWire as it’s also known, which was originally created in 1994 in its first incarnation, 1394a. Since then its bandwidth has increased from 100Mb/s to 800Mb/s, with possible transfer distances of 100m using the most recent IEEE 1394b version. In addition, this newer version could deliver power to the devices and uses cables that created less electromagnetic radiation, meaning there is less noise in the data transfer.
Unlike GigE, there is a guarantee of a certain amount of bandwidth for each device, so image transfer is never held up and it involves lower CPU usage. It is also possible to network cameras in a more economical way than is possible with Camera Link.
The IEEE 1394b stnadard used in this Sony camera stands the test of time
It seems FireWire is to be reincarnated for a third time in the IEEE 1394c standard; a white paper detailing the standard was released last year, and Destruels believes products may be available for the customer within the next three years. He believes it will marry the advantages of IEEE 1394b and GigE vision, with long distance cables, high bandwidth and improved networking abilities.
Camera Link and Power over Camera Link (PoCL)
Camera Link may be one of the oldest interfaces still being used, but it is also the fastest (until 10GigE hits the mainstream), using a high-speed serial cable and National Semiconductor’s Channel Link chip set. Another advantage is instant control of input and output, like triggered image capture for example, which are carried in a different path to the image data, meaning it has become popular with military, scientific, and medical applications. This interface does, however, require expensive frame grabbers, so it has not proved to be successful in machine vision applications, where cost, rather than the data capture interface, is the biggest factor.
That is not to say that it doesn’t have its advantages over GigE Vision, particularly when multiple cameras are used. In GigE, the cameras share bandwidth, limiting the performance if many are used. This is not the case with Camera Link, although additional frame grabbers do need to be bought, depending on the image quality required.
The latest development in the Camera Link standard, however, may be about to boost its acceptance in industrial settings. Power over Camera Link, or PoCL, is already popular in Japan, where multiple PoCL cameras are being used to inspect hundreds of products per second.
As its name suggests, the interface, like FireWire and USB, can power small cameras through the same cable as the image transfer, reducing the number of cables. This makes it ideal for applications with limited space.
PoCL makes use of another development, Mini Camera Link, an interface designed for tiny cameras only a square inch in size. The standard started development two years ago and has only just been approved with its own logo. Electrical safety was one of the biggest issues, as the power is transferred through one of the same pin outs as the existing Camera Link cables, which could be dangerous if incompatible components were connected.
To solve this, the committee developed Safe Power, so each frame grabber contains a sensor that detects whether the camera is PoCL compatible. A simple fuse, however, also provides a cheaper alternative.
It’s already popular in Japan, but it has yet to be widely accepted in Europe. Andrew Buglass, a product manager at Active Silicon, which was heavily involved in its development, believes this won’t be for long. ‘It makes a lot of sense. The cost is negligible, but it reduces the size of cameras and opens many applications. It claws back the unique points of FireWire and USB.’
It’s not just interfaces that need to be standardised; customers were finding that camera specifications were unintelligible tracts coined by manufacturer marketing departments, rather than workman-like documents coolly detailing a camera’s features.
The EMVA 1288 standard changed all this, defining a unified method of measuring imaging equipment's features, and demanding that the raw data is provided with the specifications, so the calculations can be verified. The standard is an ongoing process: it’s a year old so far and is being released in modules. Currently, standards for the measurement of noise and sensitivity, and linearity, have been described. An example could be the average number of defect pixels of the camera – in the past, this depended solely on the manufacturer’s definition, making comparison between companies impossible.