Board and backplane design is ever-changing, as designers adapt to new system capabilities and make headway in developing more. Michael Munroe, technical product specialist at Elma Electronic, talked with PD&D about how VITA design standards are affecting the architecture behind the chassis. This will be the last of several stories stemming from the Embedded Tech Trends show in January. Munroe’s fresh perspective on board and bus design began with how standards help deal with the higher speeds enabled by new developments in connectors.
It helps to know the background first: VITA 66.1, adopted in December 2011, allowed for a very popular optical ribbon connector that held one or two MT ferrules that can support up to 24 fibers each. The number of ferrules were limited, since the fibers have to be pressed tightly together in order to prevent creating a reflective surface (which creates interference). The pressure itself makes it difficult to make a system like this that works well.
At this time, the VSO is now finalizing the VITA 66.4 standard, which defines a half-size optical module with a single 24 fiber ferrule connection through the backplane. Already these VITA 66.4 optical modules are being fielded in anticipation of the completed standard.
“We needed an optical module that didn’t take up the entire J2 I/O connector in 3U VPX backplanes,” Munroe said, because pins are precious. Finding enough space between data plane connections and I/O was difficult, so 66.4 enables giving up only half of the J2 connector as an acceptable compromise.
The challenge of pushing data rates beyond 12 or 16 Gbps may no longer be practical with the existing connector system. Munroe predicts that the solution will soon be to move the highest speed signals from slot to slot using optical cables bypassing the backplane entirely. Munroe said that he thinks that in the future, the industry will see more setups where I/O goes card-to-card in speeds greater than would be possible on the backplane. These new VITA 66.1 and 66.2 blind mate optical backplane modules will support optical ribbon cables to enable efficient slot-to-slot fiber connections.
The VPX connector can actually bring more power onto a plug-in module than can be conceivably cooled by any other available cooling technique.
The primary limitation on how much power can be drawn through a connector are the dimensions of the blade and the spring arm at the interface. The density of the VPX connector makes cooling this interface a challenge because excessive heat can damage the contacts or reduce the spring force, limiting the amount of power that can be sent through the connector. With three voltage rails, over 250 watts could be brought onto a single 3U x 160 card, and nearly 400 watts onto a 6U x 160 VPX card – but again, that’s more than systems can cool.
VPX also uses very high density connectors, with up to 32 differential pairs on a 3U card.
“What every engineer wants for Christmas is more contacts,” Munroe said.
Optical connectors enable more high speed communication while at the same time reducing the design complexity of the backplane and cards, since if the signals come out of a transceiver immediately into a fiber the backplane doesn’t have to be redesigned for every application. Using optical fibers lets you simplify the design. They’re also lighter than copper, and are immune to noise from other cards.
“So the optical fibers added to this architecture means we’re no longer limited by the challenge of designing backplanes and daughter cards at higher speeds, and we’re no longer limited by the connectors themselves,” Munroe said. “Now the optical industry is looking beyond 28 Gbs to 56 Gbps per fiber.”
Meanwhile, PCIE generation 3 is also being introduced, which allows for communication at 8 Giga-Transfers/s . The PCI-SIG has also defined a new optical cabling standard which allows PCIE signals to be passed from chassis to chassis, card to card or to external I/O.
Munroe said that he expects to see more of these type of connectors, which enable going card to card at speeds higher than possible on the backplane.
One particular place where the new standards would be useful is in the defense industry, in which adding VITA 67 and 67.1 RF connectors into the VPX architecture can open up a wide range of applications. It also enables cables to be attached at the back of the card instead of the front, preventing having to go through a knot of cables to physically get to the cards.
A development on VITA 49 allowed people building for the military to define and process multiple, unique waveforms in a single chassis, allowing more space to be saved in military vehicles.
Another new addition to VPX architecture is the incorporation of radial auxiliary and reference clocks. When using these clocks for new signal processing applications, it’s desirable to have radial clocks that arrive at each card at the same time with a reference pulse that allows precision sampling. VXI and PXI enabled that type of radial clock architectures for physics and test equipment applications, but traditional backplane architectures did not generally have it. VPX has now standardized how high performance signal processing with radial clocks can be accommodated.
VITA also wants to be able to standardize edge-launch connectors, which would enable RF signals to launch off the edge of a mezzanine card in order to achieve many different efficiencies, including lower cost, reduction of flex and vibration in cables, and allowing for more custom builds.
Higher density optical connectors and optical ferrules built to manage some of the mechanical force challenges between fibers are also up and coming, Munroe said. Applications like this are used primarily in the commercial world and for ground-based communication, but as they are used more often, the defense sector might see that they are worth the expense. Modified ferrules have a lot of potential, particularly in harsh environments.
The new features described will allow the VPX architecture to deploy backplanes and systems with faster and more complex networks. The VITA Standards Organization is creating the standards that will enable these new capabilities.
Filed Under: Rapid prototyping