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Machine Vision Speeds Up
by Tom Lecklider, Senior Technical Editor
All dressed up with 680 MB/s of image data? PCIe gives it somewhere
to go.
Location, location, location may be the real estate salesman’s motto, but in
digital I/O, it’s bandwidth, bandwidth, bandwidth. Because of the ever-constant
demand for more I/O speed, it’s easy to understand the attention that PCI
Express (PCIe) has received by significantly improving on PCI’s performance.
PCIe already has become the de facto standard for graphics I/O within new PCs
and increasingly is appearing in fast data acquisition and machine vision
applications. The first PCIe frame grabber was introduced just over a year ago.
Today, there are about a dozen manufacturers producing PCIe frame grabbers with
widely varying capabilities. Currently, no cameras have been designed with a
PCIe interface although this may occur in the future.
PCI, the long-established PC bus standard, uses a shared, 47-pin, 33-MHz, 32-b
parallel bus and can achieve a 132-MB/s data rate. The fastest PCI-X variant
with a full 64-b implementation and a 133-MHz clock reaches a 1-GB/s rate. In
contrast, a PCIe lane uses two two-wire, 2.5-GHz, low-voltage
differential-signaling (LVDS) serial buses—one for each direction of traffic.
Data and control signals are sent as packets, eliminating the need for
additional wiring.
Currently, ×1, ×4, ×8, and ×16 versions are available, making possible near
4-GB/s bandwidth. Even the ×1 bus boasts a 250-MB/s capacity, more than
sufficient for many medium-speed cameras. Also, PCIe lanes are dedicated
peer-to-peer links, meaning that bandwidth is not shared.
There’s no doubt that adopting PCIe has eliminated or greatly reduced the I/O
bottleneck presented by PCI. However, you may question whether the problem of
restricted bandwidth simply has been pushed farther along to another part of the
PC system. Indeed, you can’t present a 4-GB/s data stream to a single hard disk.
Instead, very high data rates require a redundant array of independent disks
(RAID) system with the necessary bandwidth, memory buffering, and capacity.
Fully buffered dual in-line memory modules (FBDIMM) also are a fast storage
solution. Chuck Petersen, lead designer at EPIX, commented, “Motherboards now
can accept up to three PCI ×4 boards. We have tested motherboards that
simultaneously handle two 625-MB/s image data streams. One board had eight slots
for FBDIMM and was tested with 6 GB of memory. Further testing with up to 16 GB
of memory soon will be undertaken. PCIe is one part of coping with very
high-speed image data, but a 64-b operating system also is important. By fully
supporting 64-b Windows XP, we can capture data into 16 TB of memory.”
The future availability of an enhanced version of PCIe with twice the present
bandwidth is yet another reason that this bus is rapidly gaining support.
However, not all applications require blinding speed, and there are several ×1
frame grabbers that can multiplex data from as many as four cameras. In American
ELTEC’s PC_EYE/ASYNC, for example, each of four simultaneous monochrome analog
channels can be digitized with 8-b resolution at up to 40 MS/s. The data is
stored in separate memory regions and transferred under DMA control via the PCIe
bus.
How’s That Grab You?
There are a number of features that vendors emphasize to differentiate their
frame grabbers from the competition:
• Speed
• Price
• Number of Cameras Supported
• Onboard Processing
• Onboard Image Memory
• Triggering
• Camera Controls
• Reconfigurability
• PCI Compatibility
Your application and budget will determine which of these factors are most
important to you. Generally, it’s not necessary to consider more than a ×1 board
if you are using analog cameras. As the American ELTEC example demonstrated, a
single-lane PCIe product has more than enough bandwidth to support four
independent cameras.
In fact, Leutron Vision and dPict Imaging also offer products in a PCI version
as well as single-lane PCIe. Their rationale is to maintain compatibility when
PCIe interfaces replace PCI. Obviously, these products only can achieve data
rates that PCI can support, but many applications fit well within that 132-MB/s
limit. In addition, Leutron Vision provides a range of PicPort®-Exp-CL-Stereo
Frame Grabbers with 800-MB/s sustained or 1-GB/s burst data transfer rates via a
PCIe ×4 link (Figure 1).
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Figure 1. PCIe Frame Grabber Architecture
Source: Leutron
Vision
(click
here to see larger image) |
The comparison chart that accompanies this article lists PCIe frame grabbers
from a number of companies. Those products listed are compatible with Camera
Link cameras or professional analog cameras.
Stages of PCIe Adoption
There are at least two levels of PCIe implementation represented in the
comparison chart. The most expedient approach and perhaps least expensive in
terms of design time is to replace a frame grabber’s present PCI interface with
a PCIe interface. As demonstrated in the Leutron, dPict, and American ELTEC
examples, the board remains limited to the speed of the present interface.
If PCI-X has been used, speeds from 500 MB/s to 1 GB/s may be available. In
these cases, a ×4 PCIe implementation provides sufficient speed and a much more
robust interface by avoiding 64-b bus width and 133-MHz clocking. Of course,
because the design modifications don’t affect more than data output-related
circuits, this group of frame grabbers tends to perpetuate the features of the
former PCI product in the PCIe version.
The second, more aggressive approach is to add features that PCIe’s high speed
makes practical. An example is the EPIX line of PIXCI® frame grabbers. According
to the PIXCI E1 data sheet, images are stored in motherboard memory or a RAID
array.
Adopting a similar approach, BitFlow has developed FlowThru technology, and
Active Silicon emphasizes onboard FIFO buffers rather than frame storage. These
products do not have large amounts of onboard frame storage memory, instead
offloading that cost to image storage in the host.
 These frame grabbers capitalize on PCIe to reduce the need for large temporary
memories on data acquisition products. In addition, as the Active Silicon
Phoenix data sheet provided, “The majority of the functionality is implemented
in a single field-programmable gate array (FPGA), providing a flexible solution
for interfacing to Camera Link-compliant sources. The FPGA implements the PCIe
interface, hardware scatter-gather control of DMA, acquisition control, region
of interest and subsampling control, data mapping functions, data path FIFOs,
and counter/timer support.”
It’s not possible to say that EPIX and BitFlow also implement these functions
within an FPGA, but both companies’ products use a large FPGA and offer similar
functionality. BitFlow has gone farther than its competitors in describing
Karbon-CL as a platform for virtual frame grabbers. The Karbon-CL can be
customized by BitFlow to suit an OEM customer’s needs. Only a few manufacturers
support direct customer customization of the FPGA-based functions.
Twilight for Frame Grabbers?
PCIe proponents are not about to confine the bus to PC enclosures.
Experimentally, PCIe connectivity via several meters of cable has been
demonstrated, and some industry experts think that’s only the beginning. In
fact, there’s a standardization committee within the PCI Special Interest Group
(SIG) that is working on the PCIe external-cable specification.
With cabled PCIe in mind, EPIX’s Mr. Petersen forecasted, “The future will see
cameras with PCIe built in and motherboards with PCIe cable and no slots. This
will allow smaller and cheaper high-bandwidth computers to support the high data
rate of parallel output cameras. By putting the data reduction inside the
camera, the cost of the system will go down.”
If you consider EPIX’s high-speed image acquisition experience, Mr. Petersen’s
comments can’t be ignored. Nevertheless, contrasting opinions also have been
expressed, for example, by GigE camera proponents such as Pleora Technologies.
These companies are establishing their cameras as viable alternatives to Camera
Link-compliant products. GigE cameras interface to standard network interface
cards (NICs) in the host PC and don’t use a separate frame grabber. Instead,
frame-grabber functions are integrated into the camera. This is the type of
product Mr. Petersen is referring to, except with a PCIe interface rather than
GigE.
Please click here for
Machine Vision Chart.
In terms of speed, the 1-Gb/s rate of GigE is equivalent to about 100 MB/s,
significantly below the 250-MB/s rate of a ×1 PCIe link. However, 10GigE is on
the way, and upgrading from GigE to 10GigE is expected to follow the low-cost
model of previous Ethernet speed improvements. 10GigE will directly compete with
×4 PCIe performance.
Alain Rivard, the vice president of R&D at Pleora, presented his company’s point
of view: “As the bandwidth requirements of vision applications grow, speeds
greater than 1 Gb/s will be required. When this happens, PCIe cameras, which do
not yet and may never exist, might be more attractive than Camera Link cameras
because a separate frame grabber would not be required. However, PCIe cameras
are not likely to be a good solution because they will be limited by short and
complex cable configurations and, like Camera Link, will not offer networking
flexibility.
“The more likely scenario is that very high bandwidth requirements will be met
by 10GigE cameras,” he predicted. “These cameras will work in conjunction with
the PCIe bus to deliver a high-bandwidth pipe straight to memory without a frame
grabber while at the same time offering the long-distance reach and networking
flexibility of all Ethernet systems.”
Mr. Rivard’s comment about combining Ethernet and PCIe already is happening in
GigE systems. Pleora has introduced the eBUS Optimal Driver that supports
Intel’s PRO/1000 NICs and 825xx NICs, both of which interface to the PCIe bus.
This driver streams video data directly to user memory in real time, bypassing
delays possible in the Windows/Linux IP stack.
 Agreeing that PCIe may not challenge present camera interface standards in the
near future was Steve Kinney, product manager at JAI PULNiX: “Camera Link
addresses not only image transfer, but also real-time signaling to the camera,
bidirectional communications, and signaling from the camera to the host.
Although PCIe obviously is very fast and could be part of a new camera standard,
a lot of work would be required in these areas to provide a complete camera
interface solution.
“In addition to the lack of definition for direct use in cameras, PCIe has
physical problems such as large connectors not ruggedized for an industrial
environment in contrast to Camera Link’s very small and robust connectors. In
addition,” he continued, “the Channel Link chipset costs only about $1.25 and
directly handles camera data. PCIe and other serial standards such as GigE and
Firewire require additional hardware for data packaging and flow control.
Finally, look at the very large installed Camera Link base.”
Mr. Kinney’s sentiments were echoed by Inder Kohli, product manager at DALSA
Digital Imaging, and by Dwayne Crawford, product manager at Matrox Imaging. Both
stressed that PCIe already is serving a critical role in high-bandwidth image
acquisition systems by breaking the PCI or PCI-X bottleneck. Rather than
predicting that PCIe will become part of the camera interface, these managers
see Camera Link having a strong foothold that PCIe helps enhance as a very
high-speed frame grabber-to-memory bus.
Mr. Crawford suggested a possible market segmentation model based on triggering,
I/O determinism and jitter, cable length limitations, transfer speed, and the
available software development kit for a given interface (Table 1).
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Table 1. Market Segmentation Model |
PCIe
Yet another point of view was expressed by Kyle Voosen, National Instruments’
machine vision product manager, “NI has recently introduced PCIe frame grabbers
for Camera Link, GigE, and IEEE 1394 camera standards. It’s obvious that PCIe’s
very high bandwidth helped users realize the full potential of 680-MB/s Camera
Link cameras. In IEEE 1394 and GigE systems, PCIe enabled tasks beyond vision
such as motion control, data acquisition, and industrial communications.
“There is talk in the industry of PCIe being used directly for high-end
machine-vision applications. Nevertheless,” he continued, “there is no crystal
ball that will show us which standards will dominate in the future. One thing
that does seem clear, however, is that GigE will displace analog cameras, which
traditionally have been the only option when long cable runs were required.”
Summary
If you need a high-speed vision solution or are contemplating upgrading to PCs
with PCIe slots, the frame grabbers listed in the comparison chart may be
applicable. Alternatively, perhaps the cable length and networking opportunities
a GigE camera and host NIC offer are more attractive than a Camera Link
approach. Or, maybe analog cameras have all the performance you need.
As with all system design, vision applications often have several possible
solutions, each with its own set of constraints. Particularly when cameras are
used to measure moving objects, synchronization is important. This may involve
complex triggering, quadrature encoders, or special-purpose I/O lines and is an
area that frame grabbers address well. Some integrated solutions are more
limited in this regard.
One limitation of very high-speed integrated cameras is power. A separate frame
grabber is not restricted to the three to four watts of a typical small
form-factor camera. A PCIe frame grabber can perform complex on-the-fly data
manipulation without regard to the additional few watts such high performance
may require. Of course, neither 10GigE nor PCIe cameras are yet available, and
high-speed, low-power digital technology may have solved this problem by the
time they are.
Frame grabbers and cameras that emphasize high-speed direct-to-memory operation
typically perform an amount of processing on data before transferring it to the
host memory. However, should your system need additional processing, it must be
done by the host PC. Alternative frame-grabber implementations with onboard
frame memory and programmable processing may provide the functionality you
require without encumbering the host.
In terms of cost, EPIX’s Mr. Petersen ranked solutions this way: “The
lowest-cost analog interfaces still are RS-170, NTSC, PAL, and CCIR. USB is the
lowest-cost digital camera interface, followed by Firewire. GigE is the next
most expensive and will work for 75% of camera bandwidth requirements.” (Note
that Pleora’s Mr. Rivard claimed 90%.) “Camera Link will be at the upper end,
but 10GigE will appear soon.”
The good news is that you have more machine vision solutions available with the
introduction of several PCIe-based frame grabbers. Only a thorough review of
your system requirements will determine which, if any, is best for you. |