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Signals Take Backplane Routes

Thu, 10/25/2007 - 6:14am
Jon Titus, Senior Technical Editor

titusBackplanes serve as the central nervous system in a great many electronic products from industrial controls to telephone exchanges. In a typical backplane, front-side connectors hold boards in place and conduct signals from board to board and to I/O connectors. Without carefully designed backplanes, engineers could not extend the state of the art in computer and communication equipment.

Engineers Can Start with the Standards

 
Backplane manufacturers offer a variety of products for standard buses as well as backplanes designed for the customer. The latter products can accommodate special I/O cards, power supplies and on-board circuits. Courtesy of Vector Electronics and Technology.
Backplane manufacturers offer a variety of products for standard buses as well as backplanes designed for the customer. The latter products can accommodate special I/O cards, power supplies and on-board circuits. Courtesy of Vector Electronics and Technology.
Engineers can buy off-the-shelf backplanes for a variety of standard buses such as VME, VME64, VME64x, CompactPCI (cPCI), PCI Express and even for an older bus such as STD-32. If necessary, they can customize a standard backplane or create one of their own.

"Most of the customers we deal with want a standard backplane," said Jerry Rodriguez, vice president of operations at Vector Electronics and Technology. "Frequently, we sell backplanes for the VME and cPCI buses already installed in our racks and enclosures so engineers do not have to worry about packaging. Our cPCI backplanes adhere to the PICMG 2.0 Revision 3 standard, and we have an eight-slot backplane for the PICMG 2.16 1000BaseT packet switched bus." Engineers involved with prototypes and development work can buy uncommitted backplanes that let them wire board-to-board connections as needed.

Rodriguez works with customers to determine the type of bus and the size of plug-in cards that best meet a system's requirements. Backplanes from Vector range from 56-contact card-edge-connector buses to 96-pin DIN backplanes (VME or uncommitted) on to VME64x backplanes with 160 pins. If engineers need still more pins, a cPCI backplane might meet their needs.

Signals Can Take the Back Route

Engineers also should know whether they need access to I/O signals on the rear side of the backplane, explained Rodriguez. "They can use a connector shroud on protruding pins on the P2 VMEbus connector and route I/O signals through the back of their enclosure. If they need more I/O connections, a 6U cPCI card gives them access to I/O signals at the P3, P4, and P5 connectors. Commercial cPCI cards often bring out I/O signals on these connectors."

 
This backplane provides 16 slots for ATCA cards in a format that fits in a European Telecommunications Standards Institute (ETSI) rack that measures 600 mm across. Courtesy of Elma Bustronic.
This backplane provides 16 slots for ATCA cards in a format that fits in a European Telecommunications Standards Institute (ETSI) rack that measures 600 mm across. Courtesy of Elma Bustronic.
"Think about your plug-in board thickness, too," cautioned Rodriguez. "All of our card cages that comply with the IEEE-1101.10 specification can accommodate a board up to 0.090¨ thick. Greater thicknesses may require custom card guides in an enclosure."

Some systems require a custom backplane because designers plan to include proprietary boards in a system or they have non-standard signal and I/O needs. Carlo Gavazzi and Elma Bustronic represent the types of companies that produce custom-backplane products.

"We stress that designers should work with a supplier that already has experience producing backplanes that comply with a standard such as VME, cPCI, ATCA and others,” explained Steve Corbesero, vice president of engineering at Carlo Gavazzi. "The ATCA standard, for example, provides for a switched-fabric topology, so the backplane must provide the signal-integrity environment that lets the differential serdes (serialize/de-serialize) devices function properly and lets the backplane fabric achieve the bandwidth specified by the ATCA spec. You must have experience with a high-speed low-voltage differential-signal fabric to design a backplane for it."

Maintain Your (Signal) Integrity

Backplane design also requires a thorough understanding of signal-integrity issues. Thus, backplane vendors should have tools that let them simulate high-speed serdes signals with pre-emphasis and de-emphasis that compensates for any signal distortion caused by transit through the backplane, said Corbesero.

"We attend many of the interoperability workshops’ "plugfests" that let us test backplanes and boards with those from other manufacturers," noted Corbesero. "Companies can test and characterize backplanes and boards on their own, but few companies can amass all the test equipment and the variety of products available at these meetings. These gatherings let vendors ensure their products comply with industry standards."

 
Backplanes come in many sizes. This example produced for a military contractor accepts 14U cards that measure approximately 59 cm high (23¨). Courtesy of Elma Bustronic.
Backplanes come in many sizes. This example produced for a military contractor accepts 14U cards that measure approximately 59 cm high (23¨). Courtesy of Elma Bustronic. 
Based on his experience, Corbesero outlined four traps designers can fall into when they need a custom backplane:
  • Lack of sufficient signal-integrity analysis.
  • Failure to take into account the effects of EMI/RFI and EMC on a design, and failure to consider EMI/RFI produced by a system.
  • Disregard for cooling in the entire system. "Engineers must run a thorough thermal simulation of their enclosure, boards, and backplane to ensure they allow for proper cooling," stressed Corbesero.
  • Incomplete understanding of cabling and backplane access. Designers must determine how they will use rear-transition modules that connect I/O signals to backplane connector "tails."  

 
The rear view of Tracewell Systems' T-Frame for cPCI shows a standard 8-slot cPCI backplane with I/O connector shrouds, rear transition-module card guides, and direct connections to the company's power backplane for one or two power supplies.
The rear view of Tracewell Systems' T-Frame for cPCI shows a standard 8-slot cPCI backplane with I/O connector shrouds, rear transition-module card guides, and direct connections to the company's power backplane for one or two power supplies.
Justin Moll, director of marketing at Elma Bustronic, also stresses the need to carefully investigate signal-integrity issues that include crosstalk, skew, and propagation delay. "Engineers can do some signal-integrity analysis and get a post-layout analysis of the design," said Moll. "When creating a net-list or schematic, they should try to keep the spacing of the traces in mind and prevent a "rat's nest" of pin connections grouped together. Simulation can help predict performance so engineers have a reasonable level of confidence before they commit to a design. Then when they get a backplane prototype, they can test it and compare actual performance with that they expected."

"If engineers need to customize a cPCI backplane, for example, most likely they will not need simulations, although we can critique their design and offer suggestions," said Moll. "When they get into complex and higher-speed architectures, they should establish a 'partnership' with a vendor and work closely with the vendor's backplane engineers to get the performance they need. A big chasm exists between trying to produce a backplane from an open spec on your own and working with a vendor who already has experience designing backplanes for that spec."

Specialized Backplanes Require Specialized Parts

Moll stressed a key point designers may not consider: Custom backplanes often require specialized connectors and components. "Know the lead-time for specialized or customized parts so that you and the backplane vendor can plan accordingly."

During the design of a backplane, you may find some components you plan to mount on the backplane PCB do not have long enough leads to pass through your multilayer design. So, you may have to balance component characteristics with backplane thickness, and thus its number of layers. Keep in mind as a backplane's thickness increases, so does its cost.

Layer thickness also plays a role in signal integrity. "Thicker boards usually lead to longer stubs in vias," explained Moll. "At frequencies above about a gigahertz, long stub lengths have increasingly greater signal degradation than shorter stubs."

Many of the customers Tracewell Systems works with require complete custom systems that include semi- and full-custom backplanes. Along with its line of standard backplanes, the company designs and manufactures custom backplanes, or contracts with vendors with the necessary skill set, based on business needs and requirements of the project.

Make Sure Your Signals Can Get Through Customs

 
A customized PXI backplane for a military test application provides precision clock circuits directly on the rear side of the backplane and cPCI bridges on mezzanine cards. Courtesy of Tracewell systems.
A customized PXI backplane for a military test application provides precision clock circuits directly on the rear side of the backplane and cPCI bridges on mezzanine cards. Courtesy of Tracewell systems.
As both a designer and specifier of custom backplanes and a system integrator, David Angelo, senior applications engineer for the sales and marketing group at Tracewell Systems, offers advice to engineers about to design or specify their own custom backplane. "First, know the nature of all of your signals and power sources. People may want gigabit Ethernet I/O on standard VME or VME64x backplanes, for example. They could end up with two signal paths with different characteristic impedances on the same layer. If they do not point that out, their backplane vendor probably will not catch it and will not treat the paths accordingly.

Second, said Angelo, know your power requirements. Some backplane vendors do not like to mix 1/2- or 1-oz. signal traces on a layer that will include 3- or 4-oz. power planes. That situation arises when designers run out of PCB layers. Not all vendors feel comfortable with different copper "weights" on one layer.

Third, know backplane thickness specifications. Vendors may increase thickness to maintain characteristic impedances or to meet power requirements. But a thicker backplane means shorter connector tails on the back of board connectors. VME, VME64x, and VXI connectors do not offer a wide selection of tail lengths for I/O connections. A difference of 0.040¨ can spell success or failure for I/O connections.

 
 A backplane for the Advanced Telecom Computing Architecture accepts two switch cards in the center slots with the large connectors and 12 node cards. Carlo Gavazzi offers this backplane in either a dual-star or a full-mesh format. 
A backplane for the Advanced Telecom Computing Architecture accepts two switch cards in the center slots with the large connectors and 12 node cards. Carlo Gavazzi offers this backplane in either a dual-star or a full-mesh format.
Fourth, carefully describe non-standard I/O voltages. If you have telecom signals that include a ringing voltage, for example, the backplane must meet specific conductor creep and clearance specs to meet U.L. requirements.

Finally, do not skimp on ground planes and decoupling capacitors.

Backplanes may seem like ho-hum signal carriers, but engineers must consider them a key component and a high-tech part of a system's design. Without a backplane that can faithfully transfer signals among boards, system performance suffers or systems simply fail.

For Further Reading

Gavril, Bogdan, "Maintaining channel compliance in high-speed backplanes," EDN magazine, October 12, 2006. www.edn.com/article/CA6378092.html

Heister, David, "Trends in High-speed Backplane Design," RTC Magazine, January 2007. www.rtcmagazine.com/home/article.php?id=100783

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