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Backplanes face dead-end at 56G

Posted: 02 Feb 2015     Print Version  Bookmark and Share

Keywords:Rambus  backplane  Ethernet 

The rising demand by big data centres and carrier networks for faster and more efficient interfaces is driving the need for speedier computers and communications boards. Suffice it to say that existing technologies may be inadequate to accommodate this ballooning need for speed.

A senior engineer from Rambus Inc. revealed that the problem hits at the 56Gb/s speeds needed to drive systems that cost-effectively deliver 400Gb/s Ethernet. But engineers are finding they just can't drive signals at 56G across boards with two connectors that have traces typically up to 40in long.

"That's out of the picture for now, something has to change," said Wendem Beyene, a senior principal engineer at Rambus.

Wendem Beyene

Wendem Beyene revealed that the problem hits at the 56Gb/s speeds needed to drive systems that cost-effectively deliver 400Gb/s Ethernet.

Beyene showed results of research sending 56G signals between chips and modules and even between boards at distances up to 20in. The work between chips and modules used conventional NRZ signaling and four-tap FFE equalisation.

As the lengths reached 20in, Rambus researchers had to either adopt PAM-4 signaling or five-tap DFE equalisation, both requiring more complex and expensive chips. All the boards used relatively new Megtron-6 HLVP board materials with foil finish back drilled vias and the latest low-crosstalk connectors available.

Beyene said he holds out hope for a new generation of low-loss board materials that could emerge for the first time at a May event that often draws top suppliers from Japan. "There are different opinions. We worked hard to get Megtron-6 into the production flow, so some people are more interested in the option of adding re-timers. We are still trying to get information about the price differences at low and high volumes."

Companies are understandably shy about shifting materials. The 28Gb/s boards made for today's systems were some of the first to use Megrton-6. The prior 10Gb/s generation was among the first to go off mainstream FR4 boards, adopting Nelco 4000.

"My sense is we may need new materials again, but many people don't see it that way," said Beyene.

Another engineer suggested assumptions about crosstalk are a wild card in the debate over NRZ and PAM-4 signaling. NRZ has been the work horse signaling technology for many years. Experts have been saying for at least a year that NRZ will not be able to stretch to use in 56G backplanes.

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Rambus found boards driving 56G signals 20in across one connector needed either PAM-4 or five-tap DFEs.

- Rick Merritt
  EE Times





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