With recent additions to the official specification plus multiple customized variations to extract more performance, PC/104-like architectures continue to be prime candidates for rugged and space-constrained embedded designs. These architectures could face troubled waters in the future, however. PC/104 has been a flexible and rugged favorite of the embedded-system industry since its introduction, allowing designers to choose from hundreds of off-the-shelf processor and expansion boards coupled with widely available desktop software to simplify system integration. Unfortunately, as users are increasingly demanding higher data rates for some applications, industry associations have not yet agreed on a strategy for the next generation of board-to-board communications. In addition, users content with the currently available data rates also face problems as the latest processor chip sets omit support for the legacy PC/104-bus architecture.

Ampro Computers in 1987 developed the original PC/104 concept to take advantage of low-cost desktop silicon and software for embedded systems. The company derived the name from the PC and the number of interface pins on the 16-bit ISA (industry-standard-architecture) bus. The company in 1992 published the first formal specification for PC/104, which the PC/104 Embedded Consortium currently maintains. PC/104 cards employ stack-through connectors that eliminate the need for a motherboard, a backplane, or a card cage. These pin- and socket-bus connectors provide a reliable signal path even in harsh environments. PC/104 cards have four corner mounting holes for board support to resist shock and vibration. Each card measures 3.6×3.8 in., and stacked-card spacing is 0.6 in.

Although the ISA bus has vanished from the desktop, it still has advantages for embedded systems. Many embedded-system designers are happy with prior-generation processors and the extinct ISA bus. Peripheral cards are simple, low-cost, and easy to design, all prime requirements of embedded products. The relatively low speed of the ISA bus also simplifies noise and EMI (electromagnetic-interference)-protection schemes. However, the main reason for its continued popularity is the fact that a large number of off-the-shelf products employ the architecture, giving designers a wide selection. Dozens of manufacturers now produce hundreds of unique, low-cost, off-the-shelf PC/104 products (see sidebar “A new look at PC/104”).

Since PC/104’s introduction, designers have incorporated several enhancements to extend performance. The PCI (peripheral-component-interconnect) bus has effectively replaced ISA on the desktop, and it was only natural for system architects to add it to PC/104. The PCI bus brings a much higher data rate for high-performance peripherals and application-specific hardware. The PC/104 Embedded Consortium in 1997 released the specification for the PCI extension, formally known as PC/104-Plus. The specification gives board designers the choice of incorporating the ISA bus alone, the PCI and ISA buses together, or the PCI bus alone. PC/104-Plus requires a new connector, J3/P3, to house the PCI-bus pins. Because loss of board space is one of the disadvantages of the PCI upgrade, the PC/104 Embedded Consortium created the PCI-104 variation, which eliminates the ISA bus. The original PC/104 version continues to outsell both the PC/104-Plus and the PCI-104 updates.

To keep up with advances in technology and remain in tune with evolving desktop software, industry groups have put forth no fewer than three standards for the next generation of PC/104 development. These updates take advantage of the latest PCIe (PCI Express) specification, which the PCI SIG (special-interest group) defines, and USB (Universal Serial Bus) 2.0 technologies for higher data rates and improved board-to-board communications (see sidebar “PCI Express: the ideal fabric for stackable systems”). Although each of the new standards delivers substantial performance improvements in stackable architectures, the resulting products do not interoperate, and each provides varying degrees of compatibility with legacy PC/104 products.

The PC/104 Embedded Consortium agreed on the PCI/104-Express specification in early 2008 to define a standard method for using the high-speed PCIe bus in embedded-system applications. The basic PCIe link consists of two signal paths that use LVDS (low-voltage-differential-signaling) swings and constant-current line drivers to communicate at a rate of 5G transfers/sec in each direction. You can increase the bandwidth of an individual PCIe link by adding signal pairs, or lanes, until you reach the desired performance level. Although the PCIe specification defines one-, two-, four-, eight-, 16-, and 32-lane widths, the PCI/104-Express specification supports only four one-lane links and one 16-lane link.

Digital-Logic offers several PCI/104-Express cards, including the MicroSpace MSM200 series single-board computers (Figure 1). The modules come with the Intel Atom processor operating as fast as 1.6 GHz plus several options for onboard RAM. The modules target applications in battery-powered mobile computers, information terminals with video displays, game systems with music output, measuring instruments, and telecommunication devices. Besides the fast CPU, the MSM200 provides all standard PC interfaces for such demanding applications, including Ethernet, an audio controller, four RS-232 interfaces, and two serial- and one parallel-disk interface. Prices for the MSM200 start at 364 euros (approximately $520) per unit (100).

Taking a different approach, a recently formed industry trade group, the SFF SIG (Small Form Factor special-interest group) in early 2008 defined the Express104 PC/104 extension. These boards incorporate one or two newly developed 52-pin SUMIT (stackable-unified-module-interconnect-technology) connectors. One of the connectors provides two one-lane links and one four-lane link, plus three USB 2.0 interfaces, a low-pin-count bus, dual SPI (serial-peripheral-interface) channels, an SMBus (system-management bus), and a set of ExpressCard interface signals. The optional second connector provides another set of one- and four-lane links. Express104 also supports an optional configuration that includes a PCI-to-ISA-bridge chip to retain compatibility with legacy PC/104 boards. Although several manufacturers have shown interest, none had publicly announced Express104 modules as of late 2008.

Offering yet another approach to enhancing the communications protocol, Micro/sys Embedded Systems created a new stackable architecture that it based on the PC/104 form factor. StackableUSB employs USB and retains the size and stacking advantages of PC/104. StackableUSB supports as many as 16 peripheral boards, takes advantage of USB plug-and-play features, and eliminates the need for a cable with a built-in stack-through connector. Micro/sys recently introduced the SBC1626 network-ready controller, which it based on the 104 form factor with seven USB ports, including five host ports through the StackableUSB connector and two client USB ports (Figure 2). In addition to its networking functions, the ARM-based SBC1626 also features 24 digital I/O lines, eight readable DIP switches, eight LEDs for application use, and four RS-232 ports; 64 Mbytes of onboard linear flash and 128 Mbytes SDRAM accommodate high-level operating systems, such as Linux or Windows CE. Prices for the basic SBC1626 start at $450 (one).

For standard performance, embedded-system designers will continue to specify and integrate classic PC/104 as long as legacy products are available. In addition, manufacturers will continue to produce these products as long as their board designers can find ways to link newer silicon to the obsolete ISA bus. For higher-performance applications, an update is necessary. Unless the industry chooses a successor to PC/104, the architecture will most likely enter a fragmentation phase, with manufacturers producing incompatible products. In the meantime, classic PC/104 lives on.