THE PERIPHERALS PLEASE
Parallel and serial ports are still standard fare on microcontrollers and on SBCs. Yet low-speed, individual peripheral interfaces for keyboards, mice, and so on are giving way to high-speed serial connections like USB and IEEE 1394.
USB leads the pack by a large margin. It simplifies SBC design since it requires only one or two USB host connections. IEEE 1394 on boards or embedded systems tends to be limited to applications that require IEEE 1394 devices like digital video camcorders. IEEE 1394b provides more robust network support and higher speeds. That said, it has yet to find its way into regular PC or embedded systems.
The one area of change is due to PCI Express. While PCI Express is the mainstay on the backplane, its ability to be easily routed outside the box suits it for a peripheral bus in many instances.
One example of moving PCI Express into the wilds outside PC is ExpressCard. ExpressCard is designed to replace PCMCIA cards. Typically, a card has one interface, but it can have both USB and PCI Express. ExpressCard hosts can handle both.
The primary limitation of the ExpressCards is that the PCI Express uses only a one-lane connection instead of the 32 maximum. Moving PCI Express outside the box usually will limit the number of lanes utilized. Nonetheless, this approach is still much more practical than PCI-X.
STORAGE
One area where migration seems to be less of an issue is storage. Parallel interfaces like ATA (Advanced Technology Attachment) and SCSI (Small Computer Systems Interface) are quickly giving way to serial interfaces, SATA (Serial Advanced Technology Attachment), and SAS (Serial Attached SCSI).
SATA's first incarnation doesn't break performance records, but its compact cabling and hot-swap capability turn it into the storage link of the future. SATA is making some impressive gains in many RAID applications, forcing many to rethink the use of SCSI and even SAS.
SAS should finally get moving this year. Its slightly faster performance compared to SATA is of little consequence. However, its SCSI functionality will be significant in replacing existing SCSI products. One saving grace versus SATA is that most SAS adapters also will support SATA drives. Likewise, a SATA cable can plug into a SAS connector.
One technology that keeps on plugging along is Fibre Channel, with speeds now reaching up to 4 Gbits/s. Large disk farms remain its primary use, limiting its interest to embedded developers.
SWITCHING FABRICS
Next to PCI Express, switch fabrics are the hottest topic with respect to buses and backplanes. Fabrics will be found in higher-end systems in the near term and will eventually wend their way into midrange systems.
StarFabric and Ethernet possess the longest track record, though 10-Gbit/s Ethernet is on the cutting edge. StarFabric represents the best alternative to Advanced Switching until the latter begins to break out of its paper standards and into real silicon. StarFabric's 2.5 Gbits/s is more than adequate for many applications. The big advantage is that StarFabric enables developers to use fabric-oriented programming techniques versus transparent bridging of PCI devices.
InfiniBand is rebounding with its 10-Gbit/s bandwidth, low latency, and ease of use. This comes on the heels of certain press outlets' words of foreboding upon learning that Intel moved from supporting InfiniBand to Advanced Switching. InfiniBand is now popping up in supercomputer designs and blade servers.
Gigabit Ethernet is appearing in many COTS fabric-based designs, but so is RapidIO. RapidIO's mix of compatible parallel and serial standards is a compelling reason to use this technology for on-board chip-to-chip communication, as well as off-board communication. Incorporating RapidIO interfaces on DSPs and FPGAs has made RapidIO the choice for computing farms.
Of all the bus and backplane technologies, switch fabrics are making the biggest difference when it comes to applications. It eliminates one of the biggest limitations on system design—how much can be placed on a single board. With fabrics, resources can be spread across any number of boards.
Old bus and board standards never die. They just get superseded. New standards are in place, and hardware is rolling off the assembly line.
See associated Table 1.
See associated Table 2.
See associated Table 3.
See associated Table 4.
See associated Table 5.
See associated Table 6.