Using CoaXPress-over-Fiber for High-Speed Machine Vision
High-speed digital imaging expands the benefits of traditional machine vision (MV), transforming it from a process control tool into a diagnostic tool. Traditional MV systems use commercial off-the-shelf imaging sensors, lighting modules and processors to guide, inspect or identify parts moving along production lines. Compared to human operators, these systems are fast, accurate and repeatable, increasing productivity in fast-paced manufacturing environments.
High-speed streaming cameras build on these capabilities to capture what traditional MV systems cannot. Thanks to their high resolution, fast frame rates and streaming capabilities, these advanced cameras enable MV in challenging applications that require real-time analysis or long record times like semiconductor inspection, 3D printing, and space shuttle launches.
These streaming cameras are just one part of the MV equation, however. To truly reap all the benefits these systems have to offer, they must be paired with cable technologies that can support transferring and processing vast amounts of image data at high speeds. This is where CoaXPress cable technology comes in.
The CoaXPress interface standard for high-speed imaging — and in particular, its recent CoaXPress-over-Fiber extension — significantly improves throughput for MV applications, all while increasing bandwidth, lowering costs, reducing system complexity, enabling recording at longer distances and much more.
An Introduction to CXP Cable Technology
As the world's leading protocol for high-speed imaging, copper CoaXPress (CXP) cable technology is a powerful standard for moving high-speed serial data from a camera to frame grabber. In addition to MV, it plays a role in many professional and industrial imaging applications like medical imaging, life sciences and defense. Hosted by the Japan Industrial Imaging Association (JIIA), CXP has been adopted as a global standard via the G3 alliance between JIIA, the European Machine Vision Association (EMVA), and the Automated Imaging Association (AIA) in North America.
Many high-speed streaming cameras use the CXP standard to maximize their usability, enabling the cameras to transfer large amounts of image data directly to back-end frame grabbers. There, the data is processed on the fly via the frame grabber field programmable gate array (FPGA) or via the graphics processing unit (GPU) that is plugged into the computer motherboard using the Peripheral Component Interconnect Express (PCI-e) bus standard. In a process called stitching, the camera divides the transmitted images by rows and then interleaves the images as they arrive in the frame grabber FPGA. Using a simple algorithm, each image is then “stitched” back together. This streaming ability avoids the time-consuming process of saving data to a camera's RAM before downloading it to a computer in long-record applications.
Under the CXP-6 standard (revision 1.1) released in 2011, each copper cable is equipped to handle data transfer rates of 6.25 Gigabits per second (Gbps) from the camera to the back-end receiver machine. On the Phantom® S990 high-speed streaming camera, for example, users can utilize up to 16 standard CXP-6 channels. The more recent CXP-12 standard (revision 2.0) released in 2019 effectively doubles this rate, making both CXP-6 and CXP-12 ideal for high-speed cameras requiring high throughput.
The more recent CXP-12 consists of a 12.5 Gbps link over a coaxial copper cable, while four CXP-12 links can easily achieve bandwidths of 50 Gbps with link aggregation. In terms of connectors, CXP-6 uses DIN 1.0/2.3 connectors with a push-pull latching system, while CXP-12 uses Micro-BNC (HD-BNC) connectors with trusted push-and-turn, bayonet-style positive locks for quick and easy connection/disconnection.
Achieving Higher Throughput with CXPoF
An add-on to the more recent CoaXPress 2.0 specification, CoaXPress-over-Fiber (CXPoF) provides a way to run an unmodified CXP-12 protocol over a standard Ethernet connection including fiber optics. In other words, it uses standard electronics, connectors and cables designed for Ethernet, but the protocol is CoaXPress instead of Ethernet or GigE Vision.
Because CXPoF combines the CXP-12 standard with optical fiber, this upgraded standard eliminates the need for multiple CXP-6 copper cables. Vision Research recently incorporated this technology into its latest MV camera, the Phantom S991, which requires just two fiber cables as opposed to the 16 copper cables required by its predecessor, the Phantom S990. The Phantom S991 includes an 8-bit output option to provide higher frame rates in the larger resolutions and reduce data throughput. It also has resolution increments of 128 (horizontal) × 8 (vertical), allowing users to maximize resolution for the required frame rate.
In terms of bandwidth, CXPoF achieves 4 × 10 Gbps on a single Quad Small Form-Factor Pluggable (QSFP+) transceiver module for a total of 40 Gbps per camera — the same net bandwidth as four CXP-12 links over four copper coaxial cables. The Phantom S991, for example, provides the same throughput as the S990, but it uses only two QSFP+ transceiver modules and fiber cables. The S991 also uses Kintex® UltraScale+™ FPGA with Euresys CXP-12 IP Core and CoF Bridge. Within the camera, this bridge converts CoaXPress packets to XGMII (10 Gigabit Media-Independent Interface) packets going toward the Ethernet Physical Coding Sublayer (PCS) or Physical Medium Attachment (PMA) block. Within the frame grabber, the bridge also converts the XGMII packets to CoaXPress packets.
The Benefits of CXPoF
The CoaXPress standard has been successful due to its unique combination of high bandwidth, low latency, high stability, and excellent reliability. When integrated into high-speed cameras, it improves the data transfer process, enabling users to obtain high-quality images at extremely fast frame rates.
CoaXPress-over-Fiber keeps and builds on these benefits. For example, because CXPoF is an add-on to the existing CXP standard, high-speed imaging experts can leverage the same programming when upgrading their camera. Because it uses Ethernet components, CXPoF also only requires standard Ethernet connectors and cables, keeping costs low. At the same time, it takes advantage of the “free” evolution of Ethernet as it progresses toward higher bandwidths — like the eventual 400 Gigabit Ethernet.
In addition to reaping the benefits associated with Ethernet, users can enjoy all the advantages of fiber optics. These cables are small and lightweight. Fiber optic technology can also handle higher bandwidths compared to copper cables, and it is immune to electrical noise — an ideal characteristic for noisy production floors and medical applications.
CXPoF opens the door to new high-speed MV applications while, at the same time, simplifying those applications where high-speed MV is already in use. For example:
CXPoF eliminates complex, error-prone repeaters. Prior to the CXP-12 standard, users had to deploy expensive repeaters to translate the CXP-6 standard into fiber, driving up costs by several thousands of dollars. This setup also created very bulky cable interfaces with many potential points of failure. CXPoF simplifies this setup, eliminating the need for repeaters and their associated failure points.
CXPoF enables recording at longer distances. CXPoF expands MV to applications requiring recording at longer distances — an undertaking that previously required the use of many costly, complex repeaters. Now, users can simply set up their MV camera up to several miles away without worrying about the added complications associated with connecting external equipment. And with the availability of single-mode transceivers and cable, there are no limits in cable length. This ability also simplifies the process of bringing MV to applications that involve ballistics or explosives, which, by their nature, require camera operators to remain far away from the recording process.
CXPoF reduces system complexity. Reducing failure points even further, CXPoF requires only two frame grabbers instead of four. Regardless of recording distance, the number of transceiver connections also drops from 16 copper cables to two fiber optic cables, creating a lightweight, stable and more mobile cable setup.
A game-changing cable technology for high-speed imaging, CXPoF technology provides many benefits to support extremely high throughput in MV applications. By combining the CXP-12 standard with fiber optic cable technology, CXPoF balances high bandwidth with low latency, and is poised for future increases in bandwidth, all while using standard Ethernet components.
This article was written by Lee Denaro, Senior Test Engineer, Ametek – Vision Research (Wayne, NJ). For more information, visit here .
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