Where Is Serial RapidIO?

Feb. 17, 2005
It's coming, according to Iain Scott, executive director of the RapidIO Trade Association. Parallel RapidIO may have carried the RapidIO flag for the past year, but developers are waiting for Serial RapidIO. Interest was evident at last month's

It's coming, according to Iain Scott, executive director of the RapidIO Trade Association. Parallel RapidIO may have carried the RapidIO flag for the past year, but developers are waiting for Serial RapidIO.

Interest was evident at last month's Bus & Board Conference in Long Beach, Calif. Serial RapidIO interfaces have been implemented in FPGAs, and a flood of native Serial RapidIO devices and switches are expected by year's end. That's when the fun starts.

Developers have a lot of faith in Serial RapidIO. Of all the switch fabrics, Serial RapidIO is the only one finding a home in processors and DSPs. Currently, parallel RapidIO is available on some processors, such as Freescale's MPC8560 PowerQUICC III. The availability of of parallel RapidIO switches makes these chips usable whereas serial RapidIO switches are currently lacking.

Most processors also will include PCI or PCI Express interfaces. However, PCI Express doesn't count as a switch fabric. Advanced Switching, based on PCI Express hardware, may change the landscape, but that's years away.

Four companies, including Tundra Semiconductor and Mercury Computers, have promised delivery of Serial RapidIO switches this year. Additional vendors are expected to follow.

Given the interest in Serial RapidIO, what's to become of Parallel RapidIO? It should remain a high-performance solution for one or two large vendors, but Serial RapidIO will likely become synonymous with RapidIO. Mixed parallel and serial implementations will be rare, so don't look for an army of serial/parallel RapidIO bridges.

On the other hand, bridging Serial RapidIO will become commonplace. The RapidFabric standard already covers encapsulation of protocols like SPI-4, and support is in the works for PCI Express. As a result, endpoints will be PCI and PCI Express devices linked to a host located elsewhere on the fabric. StarFabric does this already, and it's part of the Advanced Switching specifications.

Preparation for Serial RapidIO has progressed for all high-speed serial connections because of their similarity. For example, board standards like AdvancedTCA and the VME VXS handle all popular standards, including PCI Express/Advanced Switching, Ethernet, InfiniBand, StarFabric, and, of course, Serial RapidIO.

The same backplane can handle any fabric, though it's not possible to mix and match in the same backplane. Split backplanes are being proposed to mix interfaces, but this is typically being done to incorporate parallel buses like VME with new switch-fabric-only boards.

The dearth of SerialIO switches has deterred backplane vendors. Small development systems that use a full-mesh-interconnect, passive backplane are available from companies like Elma. Still, the key to success lies in the switch.

The only 10-Gbit/s high-speed alternatives currently available are InfiniBand and 10-Gbit Ethernet. InfiniBand has the edge on availability and cost, but there's still a window for Serial RapidIO to make a major difference.

RapidIO Trade Associationwww.rapidio.orgSERIAL RapidIO FEATURES
Multilane architecture
Like PCI Express, InfiniBand, and StarFabric, Serial RapidIO employs one or more bidirectional serial links called lanes. Serial RapidIO defines 1×, 2×, and 4× configurations.

High bandwidth
Serial RapidIO operates at a range of speeds from 1.25 GHz up to 3.125 GHz (2.5 Gbits/s) per lane.

Low pin count
Each lane uses four lines: two differential connections in each direction.

256-byte data payload
Serial RapidIO's data payload is designed to optimize overhead and minimize buffer size and delays due to switch blocking.

64k devices Serial RapidIO handles up to 64k end points—much less than Ethernet but more than enough for even extremely large embedded systems.

Traffic management Thanks to its Class of Service and Flow Control features, Serial RapidIO can handle a range of mixed traffic, from latency-sensitive audio and video streams to bursty data transfers.

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