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).

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.

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

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:

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.

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

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

Prior to the time when the embedded SBC market coalesced around the PC architecture and a handful of form-factor standards, it was nearly impossible to locate two SBCs that bore much similarity to each other. The PC architecture brought a degree of order (in several shapes and sizes) to that chaos by serving as a unifying force--a situation that persisted for nearly two decades.

Today, with the established norms being disrupted by new interfaces (USB, FireWire, Bluetooth), architectures (MIPS, PowerPC, ARM) and operating systems (Linux), the embedded SBC market had better prepare itself for a new phase of its life cycle--one that will initially be characterized by a heightened diversity of operating systems, processor architectures, peripheral interfaces and physical form-factors.

Next, we'll create some order amidst the SBC chaos by taking a brief look at available SBCs you can use in your embedded Linux-based systems.

PC/104-Plus Basic Mechanical Dimensions

Ampro's CoreModule/46E combines a full PC architecture and Ethernet interface on a PC/104-compliant module

Originally derived from the form-factor of the MiniModules used to expand Ampro's Little Board SBC, PC/104 has now become one of the most popular embedded system standards. The compact (3.6" x 3.8"), rugged, PC-compatible module standard takes its name from the self-stacking 104-pin pin-and-socket bus that contains the same signals as the ISA bus (PC bus) P1 and P2 connectors. PC/104-Plus adds the PCI bus, using a second (120-pin) board-to-board bus connector. Because PC/104 and PC/104-Plus modules tend to be made from standard PC desktop and (more often) laptop components, most manufacturers of these products claim they support embedded Linux (either directly or via a third-party relationship with an embedded Linux software provider).

By now, PC/104 is so broadly supported that it's not practical to list individual suppliers, of which there are many hundreds worldwide. Instead, we refer you to the following resources that provide PC/104 technical, product and vendor information:

Motorola has adopted EBX as an embedded motherboard standard.

The 5.75" x 8.0" Embedded Board eXpandable (EBX) specification, which was derived from Ampro's proprietary Little Board form-factor, resulted from a collaboration between Ampro and Motorola Computer Group.

As compared with PC/104 modules, these larger (but still reasonably embeddable) SBCs tend to have everything from a full PC on them, including application-oriented interfaces like audio, analog or digital I/O in many cases. Also, it's easy to fit Pentium CPUs, whereas it's a tight squeeze (or expensive) to fit a PC/104 SBC. Typically, EBX SBCs contain: the CPU; upgradable RAM subassemblies (e.g., DIMM); Flash memory for solid-state disk; multiple USB, serial and parallel ports; onboard expansion via a PC/104 module stack; off-board expansion via ISA and/or PCI buses (from the PC/104 connectors); networking interface (typically Ethernet); and video (typically CRT, LCD and TV).

Ampro publishes an EBX whitepaper and the EBX spec. Similarly, Motorola Computer Group maintains an EBX information area where you can find an EBX backgrounder and the EBX spec. See http://www.ampro.com/ and http://www.mcg.mot.com/.

As is the case with PC/104, most EBX SBC manufacturers offer support for embedded Linux on their products, either directly or via a third-party relationship with one of the embedded Linux software providers. Here are a few manufacturers of EBX form-factor SBCs that claim to support Linux:

Refer, also, to the PC/104 resources sited previously because EBX is, after all, based on PC/104 technology.

Advantech's ``half biscuit'' form-factor fits a PC, Ethernet and LCD controller in the space of a 3.5-inch disk drive.

Today, a number of companies have begun offering families of PC/104 (or PC/104-Plus) expandable embedded-PC SBCs that are approximately mid-way in size between PC/104 and EBX. Unfortunately, there are currently no standards for these ``half-EBX''-sized boards, so each supplier's product family implements a unique form-factor. Examples include products from:

Though these products lack standardization, they do offer the advantage of being roughly twice the size of PC/104 modules. Consequently, with today's very highly integrated components, they have sufficient space to fit just about all of the functions you're likely to want; yet they expand with PC/104 (or PC/104-Plus), so they are modularly expandable. Also, the increased size makes it more practical (and less expensive) to include a real Pentium-class CPU than is the case on a PC/104-sized SBC if your application happens to need one.

CompuLab 486CORE

JUMPtec DIMM-PC/486

Intrynsic's CerfBoard has built-in Ethernet and a CompactFlash expansion on its rear.

The Axis Developer Board puts the ETRAX SOC on a small embeddable board.

The design of the LART SBC is available under an open license from the Technical University of Delft (The Netherlands).

ADS Bitsy

Lineo µCsimm

Just as nonstandard SBCs are somewhat larger than PC/104 modules, a growing number are substantially smaller. Again, no form-factor standards currently exist for this category of SBCs. Additionally, many of these tiny SBCs are based on the new high-integration system-on-chip processors, which often are not x86-compatible. So, PC-compatibility tends to be the exception rather than the rule. Still, despite a lack of compatibility, their excellent integration-to-size ratios make them highly appealing for applications that can't tolerate the size of standards-compliant SBCs because they can save the costly and time-consuming task of designing a custom embedded computer.

Here is a sampling of some small, in some cases tiny, embedded SBCs (PC/104-sized or smaller) that are said to support embedded Linux:

These days, most manufacturers of PC-compatible, single-board computers claim to support embedded Linux on their products. However, be careful not to get too complacent when the sales rep says, ``Sure, we support Linux.''

In general, Linux support for PC-compatible embedded SBCs tends to be straightforward, provided the chipsets are mainstream, current and used in the normal manner. Always ask the SBC vendor what specific versions of Linux they have tested, how they conducted the tests, which interfaces on the SBC were exercised and what functions are either untested or unsupported.

Specific, areas to watch out for include:

Knowing that Linux drivers or in-kernel support exists for the chips used is encouraging but not sufficient. SBC manufacturers often take shortcuts to save money or board space and, in the process, may unwittingly sacrifice compatibility. In short, there's no substitute for testing!

Fortunately, if you are using Linux in your embedded SBC-based system, you can at least take heart in the knowledge that driver source code is readily available, and plenty of Linux-aware programmers are around who can help you untangle any problems that may arise.

In light of the rapidly diverging shapes, sizes and architectures for embedded system single-board computer platforms, and recognizing the growing importance of Linux to the embedded market, LinuxDevices.com now maintains a continually updated quick reference guide to Linux-friendly embedded SBCs that is intended to help developers locate suitable components. Visit http://www.linuxdevices.com/articles/AT2614444132.html/.