"PIXEL HIGHWAY"
Move large volumes of data efficiently with the combination of CameraLink and PCI Express
The industrial image processing application environment is increasingly developing into something like a “Metropolitan Area”. Here millions of commuters present the city's management with almost insoluble tasks in respect of the infrastructure. And what can you do to ensure that the data streams consisting of millions and millions of pixels arrive safe and sound to their goal from the peripheral, so that they can be used productively in your work on the PC? A highway using CameraLink technology is an excellent solution.
The factors bringing us to this position are well-known: the user has at his or her disposal cameras that generate huge volumes of data with their high resolution and high image refresh rates for the purposes of monitoring and controlling production processes, often using image processing systems for quality ssurance. A number of criteria define the task of optimising data transport, and hence the selection of the interface, for the user: speed, security, integration effort, flexibility and cost. A glance at the market reveals how important orientation to the correct application is.
Standards have also been defined for camera interfaces to help in integration with existing architectures. Thus FireWire™ and Gigabit Ethernet (GigE) can score here, as they bring defined software standards, the former in the shape of DCAM and he latter with GigE Vision. Standardisation affects only the hardware with USB2.0, CameraLink and analogue interfaces.
Cable length is also important in the installation of image processing solutions in a production environment. Analogue and GigE cameras perform well for lengths of up to 100 m. USB, CameraLink and FireWire™ are good below the 10 m mark. However, if we keep our focus on the facts outlined at the start, we come back again to the criteria of transmission speed and bandwidth. Clocking, processing speed and assurance of practically zero error rates in production cause us to equate the data volumes generated to the task set. The CameraLink interface now comes into play here in order not to reduce the "yield" of modern cameras.
Why choose CameraLink?
CameraLink is specified for rapid image transmission. The technology is based on the three transmission structures, Base, Medium and Full, to allow it to cover as broad a basis of applications as possible. The table below shows the specified maximum data transfer rates as a function of the clock rates. The number of taps indicates the number of channels used for the transmission of the data.
Configuration | Ports/taps supported | Number of | Data transfer rate | Maximum |
Base | A,B,C | 1 | 255 MByte/s | 85 MHz |
Medium | A,B,C,D,E,F | 2 | 510 MByte/s | 85 MHz |
Full | A,B,C,D,E,F,G,H | 2 | 680 MByte/s | 85 MHz |
For comparison, a GigE camera can transfer a maximum of 125 MB/s, FireWire™ IEEE1394b manages just 100 MB. This shows clearly that CameraLink is fully justified for fast image transmission.
CameraLink uses LVDS (low-voltage differential signalling) which is characterised by high speeds at low voltage levels as its physical layer. Up to 28 TTL signals are transmitted over standardised connectors and cables. One or two plugs must be connected for each camera, depending on the version. There is progress today on the power supply to the cameras. The introduction of Power Over CameraLink (PoCL) has significantly reduced the gap in respect of user friendliness to USB, FireWire™ and GigE, and “plug and play” is getting closer. Another advantage over these interfaces is external hardware triggering, which has a very low latency.
Bottleneck at PCI overcome
A fast camera interface is only as useful as the host bus system that passes the data on. Until a few years ago, the established standard for PC-based image processing systems was PCI (Peripheral Component Interconnect). With a bus width of 64 bits and a clock rate of 66 MHz, the maximum transmission rate of 533 MB/s is less than the maximum data transmission rate that can be achieved with a CameraLink camera configured to the Full configuration.
Significantly higher throughput rates are possible with the PCI-X standard that was agreed in 1998. PCI-X 1.0 reaches a maximum of 1.066 GB/s with a single subscriber on the bus and is thus capable of transferring the data from a CameraLink camera configured in the Full configuration without problem and, above all, securely.
The PCI Express databus, introduced in 2004 and intended for both copper lines and optical connections, is the solution to the problem. PCI Express has 1 to 16 lanes, depending on the version, and is run at a clock rate of 1.25 GHz per direction. This bus, characterised by serial point-to-point connections, is full duplex-capable. Maximum data transfer rates of 4 GB/s are achieved with 16 lanes. Using just one lane, the rate of transmission per direction is still 250 MB/s – i.e. 500 MB/s in total. A PCI Express bus should have at least 3 lanes if it is to be capable of reliably transferring data from a camera with a Full configuration CameraLink interface.
Full concentration on the job
The combination of PCI Express and CameraLink means that the user is able to implement applications previously thought to have been impossible. Vision & Control offers CameraLink in conjunction with a powerful vicosys series image processing computer. In the base level, two cameras in Base configuration or one camera in Medium configuration can be operated on the vicosys multiple camera system.
Applications in the stamping and beverages industry with typically high cycle rates also demand software algorithms which can achieve optimum evaluation of image information. Only a finely tuned hardware and software package will be able to cope with the constantly high clock speeds so that, to get back to our analogy, the traffic on the Pixel Highway always flows quickly and safely.
Download (0.12 MB)
Download flyer Vision & Control (0.83 MB)
