Neil Chen,
Product Manager, ADLINK Technology
All industrial vision appliances require both image data and a power supply, and most of the industrial cameras require two cables-one for image data transmission and the other for power. Two cables mean double the installing and maintaining wiring cost, and two times the opportunity for failure.
In the early years, analog cameras almost dominated the industrial vision market; however one of the shortcomings associated with analog cameras is electromagnetic interference, or EMI. Although analog cameras use a shielded coaxial cable for signal transport, EMI can still adversely affect the 1 volt, peak-to-peak video signal contained within it. Thus, the combination of power, and image data into a single cable was not the mainstream. Technological advances such at the introduction of digital camera standards that use Camera Link, FireWire, and Gigabit Ethernet eliminate this issue as data transmission in digital camera exists in only two voltage states, and therefore is not as susceptible to EMI as analog is. After the move from analog to digital cameras, the focus in the industrial vision market became simplifying installations, reducing space, and reducing costs.
Feeding the Demand
Enhancing the digital camera standard with the combination of the data line and power line into a single cable is a common means for feeding the demand.
Power over Camera Link (PoCL)
Camera Link was developed as an open specification for cameras and frame grabber cards for use in machine vision applications. The Camera Link interface specification was defined in the year 2000 by the Automate Imaging Association, an industry group consisting of industrial camera, cable, and frame grabber card manufacturers. Camera Link utilizes low voltage differential signaling (LVDS) technology to transmit digital data.
PoCL is a recent addition to the Camera Link specification, and it provides a single cable solution for the Camera Link digital interface and power to a camera. This is accomplished without losing backward compatibility to the present Camera Link standard. A "SafePower" mode in the PoCL standard was designed to reduce accidental connections of non-compatible products that may result in damage to cameras and/or frame grabber cards. SafePower compatibility is accomplished by incorporating a camera sensing circuit on the frame grabber that will only supply power after determining whether the connected camera is PoCL compatible.
PoCL systems and devices are required to maintain standard Camera Link data and control interfaces and each PoCL device has additional functions standard to Camera Link as described below.
In order to maintain backward compatibility, PoCL cables use the same connector and cable structure as standard Camera Link cables. Table 1 describes the difference between PoCL cables and normal Camera Link cables. Wires connected to pins 1 and 26 are insulated and used as +12 V power lines. Wires connected to pins 13 and 14 are the same drain wire and used as return and power lines.
Table 1. PoCL and Camera Link cable pin definition
IEEE 1394 (FireWire)
The IEEE 1394 (registered as the FireWire trademark by Apple, Inc.) interface is a serial bus interface standard for high-speed communications and isochronous real-time data transfers that supplies a maximum of 45 W of power. Today, IEEE 1394, along with its associated IIDC(DCAM) protocol specification developed by the 1394 Trade Association, is also widely deployed choice for industrial vision applications, the IEEE 1394b is an excellent interface for industrial vision applications offering reliable data transfer, minimal CPU loading, and single cabling. Figure 1 shows the pin definition of the 9 pin IEEE 1394b connector. VG and VP are the power pins for power transmission.
Figure 1. Pin definition of the 9 pin IEEE 1394b connector
Power over Ethernet (PoE)
Gigabit Ethernet cameras can communicate with computers via existing Ethernet technologies for industrial vision applications. PoE takes advantage of proven capabilities, such as long distance transmission and high bandwidth. Gigabit Ethernet cameras typically need a separate cable to supply the power, in addition to the data cable.
The IEEE published its IEEE 802.3a Power over Ethernet (PoE) standard to specify how to deliver power over standard Ethernet cables. This allows up to 48 V of DC power to PoE-compliant devices over eight-wire Cat 5 and Cat 6 cables.
A standard CAT5 Ethernet cable has four twisted pairs, but only two of these are used for 10BASE-T and 100BASE-T. The specification allows two options for using these cables for power:
Figure 2. Power over spare pairs
Figure 3. Power over the data pairs
This specification does not allow both sets of wires to be used - a choice must be made. The Power Sourcing Equipment (PSE) applies power to either set of wires and the Powered Device (PD) must be able to accept power from both options.
The voltage is nominally 48 V, and about 15.4 W of power is available at the Powered Device. An isolated DC-DC converter transforms the 48 V to a lower voltage more suitable for the electronics in the Powered Device, while maintaining 1500 V of isolation for safety reasons.
An obvious requirement of the specification is to prevent damage to existing Ethernet equipment. A "discovery process", executed from the Power Sourcing Equipment, examines the Ethernet cables to look for devices that comply with the specification. It does this by applying a small current-limited voltage to the cable and checks for the presence of a 25 k�[ resistor in the remote device. Only if the resistor is present is the full 48 V applied, but this is still current-limited to prevent damage to cables and equipment in fault conditions. Table 2 shows the comparison of the industrial digital camera interface standards that provide the single cable solution.
Table 2. Comparison of industrial digital camera interfaces
Combining It All
Designing, installing and maintaining wiring has always been the major cost component in vision systems. Delivering power over the data or communication cable is a very useful feature in the industrial vision market. With a single cable solution, industrial vision systems can provide simplified installation, lower maintenance, and reduced total cost of ownership. Future generations of digital cameras will likely reduce wiring requirements even further.
ADLINK's solutions for PCI Express® Frame Grabber Cards
ADLINK provides a series of PCI Express® (PCIe) products for industrial machine vision applications. The PCIe bus provides high-bandwidth and robust point-to-point interconnects, and complete software compatibility with the existing base of operating system, PCI drivers, and software. The PCIe bus also provides a dedicated link for image data transmission.
ADLINK provides several types of frame grabber cards, including:
PCIe-CPL64
The PoCL standard allows the camera link able to supply power to the camera through the Camera Link connector without losing backward compatibility with the previous Camera Link standard. This solution is particularly suitable for a small camera.
PCIe-RTV24/PCI-RTV24/PCI-MPG24
Real-time signals: No network latency or protocol overhead.
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