Don't sell the distributed backplane short. It simply isn't true that its top speed is 50 MHz. And that's generally where designers run into problems—in the 40- to 50-MHz range, with anywhere from 15 to 20 slots loaded.
Running at these higher frequencies has become important in broadband applications. Of particular interest is 155 MHz, since it's a submultiple of an asynchronous-transfer mode (ATM) rate. Because of the belief that 50 MHz was an upper bound, some designers have considered abandoning a distributed backplane approach. As an alternative, they've contemplated a more complex switched topology to achieve the required performance.
"There has indeed been a preconceived notion that TTL-oriented, distributed backplanes with from 10 to 20 cards is limited to the 50-MHz range," confirms Paul Kierstead, strategic marketing manager, Interface Products, Fairchild Semiconductor, South Portland, Maine. "But using Fairchild's GTLP (gunning transceiver backplane plus) devices, we have demonstrated that good performance with decent signal integrity can be realized well beyond 155 to 200 MHz."
Such improvements are based on development programs at the EnSigna Laboratory at Fairchild Semiconductor. At this facility, the performance of the distributed backplane technique has been extended well beyond 155 MHz. Fairchild has even demonstrated decent signal integrity at 400 MHz, using particular protocols.