[Engineering Feature]
All A-Board!
To satisfy the needs of aerospace and defense applications, compact, multicore, low-power computer boards are achieving greater levels of computational capabilities per watt.
HIGHER-SPEED SERIAL INTERCONNECTS Because SBC designers employ so many different processor, operating- system, form-factor, and design architecture approaches, standardization is nearly non-existent. However, some major standardization efforts are under way in form-factor as well as in intra-board and inter-board communications.
Not willing to wait, a new generation of serial links for SBCs is beginning to use higher-speed bus interconnects for communications. Serial bus standards like VPX and XMC reflect the growing importance of high-speed serial interfaces, particularly switchedfabric interfaces, such as PCI Express, Gigabit Ethernet, Serial RapidIO, and InfiniBand. HyperTransport is another serial bus, but it remains a chip-to-chip link even though it has been defined in board standards.
Serial buses feature wider bandwidths and greater throughputs than parallel buses. Also, they offer significantly greater performance levels and are more suited to modern SBC architectures that use multicore designs. They offer lower pin counts and hotswapping capability as well.
Yet parallel buses are scalable from 1X, 2X, 4X, and so on without the need to move up to higher-link transfer rates. Additionally, parallel buses like the VME bus, CompactPCI, Embedded Technology eXtended (ETX), PC/104, and PC PCI/SA, each with variations and alternatives, are the mainstays of bus standards for SBC communications.
VME traces back to the 1970s, when it was called the VERSAbus, and it still constitutes the lion’s share of buses for SBCs. It has continuously grown, from 32-bit, 40-Mbit/s to 64-bit, 80-Mbit/s, and to 2eSST 320-Mbit/s implementations.
According to Ray Alderman, executive director of the VME International Trade Association (VITA), VME is a billion dollar market that’s expected to grow another 10% this year. VITA has been pushing for VPX (VITA 46), which is the latest specification to gain acceptance by the SBC industry. Ratified by the American National Standards Institute (ANSI) last year, it seeks to address the needs of critical embedded-system designs.
VME addresses both the needs of older 6U form factors as well as newer 3U factors. It also complements the VME standard that dominates most available SBCs. Both VME and VPX embody baseline specifications that define mechanical and electrical parameters for SBCs.
The VPX standard can support data communications over a range of 3 to 100 Gbytes/s. It raises the amount of power handled in an SBC board slot to 115 W at 5 V, from the present 90 W at 5 V for a VME bus slot. In addition, it allows for power levels of 384 W at 12 V or 768 W at 48 V.
VPX supports existing standards for cooling SBCs, but it also provides for more stringent cooling requirements via VITA’s Ruggedized Enhanced Design Implementation (REDI), formerly known as the VITA 48 standard. And though VPX is largely compatible with VME, it has a new type of connector developed by Tyco known as the MultiGig RT2.
The 6U board includes six 16-column, seven-row connectors and one eight-column, seven-row RT2 connector. The 3U board has two 16-column, seven-row RT2 connectors and one eightcolumn, seven-row RT2 connector. The MultiGig R2 boards aren’t compatible with VME connectors, though VITA envisions the use of a “hybrid” chassis to mate with VME boards, as allowed in the VPX standard.
Designers at GE Fanuc Intelligent Platforms believe that focusing on VPX offers the best way to provide high-performance computing for present and future rugged environments. “We fully expect VME and CompactPCI to continue in production for years, but VPX is the platform of choice for many new designins,” says Richard Kirk, SBC global product manager. GE Fanuc makes a number of VPX SBCs, like the 3U VPX SBC330 and 6U SBC, both based on the Freescale 8641D Core Duo PowerPC.
The VXS (VME switched serial) bus combines the event-driven VME parallel bus with enhancements to support switch fabrics. It allows for data communications over a wide range of 3 to 30 Gbytes/s. Like the VPX standard, it can plug into VME bus backplanes.
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