All about Linux-friendly Single-Board Computers

The rapidly growing and diverging single-board computer market can be hard to keep up with. Here's some history and some help.

Early microcomputers typically consisted of a half dozen (or more) circuit boards--plugged into a backplane--that implemented the central processor unit (CPU), memory, disk controllers and serial/parallel port functions. These backplane-based microcomputers were used for data acquisition, process control and R&D projects, but were generally too bulky to be used as the intelligence embedded within devices.

By the early 1980s, integrated circuit (IC) technology had advanced to where functions that previously occupied entire circuit boards could be crammed into single, large-scale integration (LSI) logic chips. LSI chips for CPU, memory, storage and serial/parallel ports now made it practical to implement complete microcomputer systems on a single board--without backplanes. The Z80-based ``Big Board'' (1980) was probably the first such single-board computer (SBC) that was capable of running a commercial disk operating system (CP/M).

The Embedded SBC Market

Like the Big Board, the ``Little Board'' (Ampro, 1983) used a Z80 CPU and was targeted specifically at the CP/M operating system, but it was much smaller in size, matching the footprint of a floppy disk drive (5.75" x 8.0"). Thanks to its unique combination of compactness, simplicity, reliability and low cost, the Little Board made it practical for a commercial disk operating system to be easily embedded directly within devices that were not themselves computers.

Thus was born the embedded SBC market, which by now has become crowded with hundreds of SBC manufacturers producing thousands of different SBC products that target a vast array of embedded and dedicated computing applications.

Initially, every SBC product was completely unique--both architecturally and physically. This was largely due to the inherent diversity of embedded system requirements, combined with the wide assortment of processors and peripheral controllers that were available. Moreover, there were no standards to influence SBC developers' choices of functional and mechanical specs.

The Dawn of the PC-Compatible SBC

By the mid-1980s, there was growing interest in IBM-PC compatibility in embedded and other nondesktop applications, for two key reasons:

  • Hardware leverage--PC chipset and peripheral compatibility could produce systems that were cheaper, simpler and easier to support.

  • Software leverage--PC compatibility could make it possible to take advantage of their operating systems (first MS-DOS, then Windows), languages, tools and application software.

Some of the resulting PC-compatible microcomputers were based on the form-factor of the IBM PC (ISA bus) plug-in card. Others were implemented as standalone (nonbackplane) systems on a single board. Still others were adaptations of popular industrial backplane buses (STD, VME).

In the case of embeddable-nonbackplane SBCs, the trend toward PC-compatibility quickly became a stampede. Consensus also emerged around several popular form-factors:

  • Little Board (5.75" x 8.0")--complete systems on a single compact board, expandable with plug-on function modules.

  • ISA ``slot boards'' (full-length, 13.8" x 4.8"; half-length, 7.1" x 4.8")--SBCs in the IBM PC plug-in card format, which, though backplane-oriented, could also function as standalone SBCs (without backplanes).

  • PC/104 modules (3.6" x 3.8")--compact, rugged, self-stacking modules featuring a reliable pin-and-socket, board-to-board expansion bus. With the coming of PCI, these were joined a decade later by the PC/104-Plus, which consisted of PCI being added to the PC/104, and the EBX, which was a PC/104-Plus added to the Little Board.

Not all SBCs jumped on these popular form-factors. Nor did they all go the PC-compatible (x86/DOS/Windows) route. Throughout the multidecade history of single-board computers there have been, and continue, to be, nonstandard board sizes and processor architectures that target unique application requirements or fill niches not well matched to the standardized form-factors and popular ``Wintel'' (PC-compatible) architecture.

Strong Winds of Change

Today, several significant factors seriously challenge the SBC market status quo:

  • Exploding demand for embedded intelligence--even the tiniest and least expensive products and devices are now expected to have at least rudimentary embedded intelligence. Many also require user-friendly graphical and/or speech interfaces.

  • Ubiquitous connectivity--there is a growing need for everything electronic to be interconnected, whether wired or wireless. These devices must often be capable of inbound or outbound internet connectivity and must support numerous standardized protocols (such as TCP/IP, PPP, HTTP, FTP).

  • Evolving peripheral and bus interfaces--although popular interconnection standards can sometimes seem immortal (consider Centronics and RS-232), new interfaces do gradually supplant the old. Nearly two decades after the birth of the PC, the ISA bus has finally been replaced by PCI. USB is now replacing the venerable serial, parallel and PS/2 ports. Ethernet is everywhere and FireWire (IEEE-1394) is beginning to make a strong showing. SCSI never made it to the mainstream in PCs (other than the Apple). We may well stand on the verge of backplane-free systems whose only expansion mechanism is via medium- and high-speed serial interfaces (USB, IrDA, FireWire, Ethernet, etc.).

  • Application-oriented system-on-chip processors--numerous highly integrated ARM, MIPS, PowerPC and x86-based one-chip systems are being developed to match the specs of a wide array of high-volume and cost-sensitive appliance-like products. Today, these ``application-on-chip'' processors represent tantalizing fodder for a new breed of high-integration, high-performance and highly cost-effective SBCs. Many of these SOCs have abandoned x86 compatibility for the sake of cost/power/integration benefits.

  • Embedded Linux--in just a few short years, Linux has exploded onto all aspects of the computing scene, offering a low-cost, open-source solution with strong support for open standards, networking, communications, Internet, graphics and more. Despite its origins as a UNIX clone for PCs, Linux now supports as broad a range of processors as any traditional embedded OS. Consequently, full-featured OS support for diverse architectures (beyond x86) has increased dramatically in the last several years, due to the rapidly evolving capabilities and growing architectural neutrality of Linux, resulting in a more level playing field among competing processor architectures.

Considering all these factors, it becomes evident that conditions are ripe for change in the embedded SBC market.