PC/104 continues to stack up

When IBM introduced its PC-AT desktop computer in 1984, few engineers would have predicted the popularity of the Industry Standard Architecture (ISA) bus used to expand the computer’s memory and I/O functions. Yet that 16-bit bus, formally IEEE P996, continues to enjoy widespread use. Many vendors of boards meant for use in embedded computers have adopted the ISA bus as embodied in the popular PC/104 standard that dictates board size and a 104-pin ISA-bus expansion connector.



The PC/104 Consortium, which oversees the standard, has not remained focused on only one type of computer board. Over the last few years, the original PC/104-size boards (3.6” × 3.8”) spawned the larger embedded platform for industrial computing (EPIC) boards and embedded board expandable (EBX) boards that also provide a PC/104 connector for I/O devices. (The PC/104 bus generally operates as a 16-bit bus, although it can serve as an 8-bit bus, too.)



Because PC/104 cards stack one on top of the other, they let engineers create compact embedded computers. “The small PC/104 boards are relatively inexpensive and vendors offer many types of SBCs and I/O boards,” noted Earle Foster, vice president of sales and marketing at Sealevel Systems. “Boards that use the PC/104 ISA bus are about all you need in an application until a system requires high-speed network communications. Even though the PC/104 specifications now include other signals, the majority of applications still seem to use the original PC/104 ISA bus.”

Foster also noted that the PC/104 connectors and arrangement of stacked boards provide a rugged computer that works well in equipment that experiences high vibration levels. “You see that type of environment in military and transportation applications,” said Foster. “Engineers might mount a PC/104-based system in the back of an emergency vehicle or police car, for example.”



THERE’S ROOM FOR EXTRAS

Engineers also like the expansion capabilities of a PC/104-based computer. “Although an SBC may have a variety of I/O ports and devices, a design always seems to require something extra,” said Bob Burckle, vice president at WinSystems. “You may need a USB port, a serial port, or a custom board of your own design. Engineers realize marketing people will add a last-minute requirement, so the PC/104 bus gives them the capability to easily update a design.” And because so many PC/104 SBCs employ X86-family processors, engineers can use Windows, Linux, or a real-time operating system (RTOS) in their embedded computer.



The PCI bus — made popular by its use in desktop PCs — has added to the capabilities of PC/104-based computers. And because the PCI bus operates with a higher bandwidth than that of the ISA bus, engineers who need high-speed communications will prefer SBCs based on the PCI specification named PCI-104.



“The bandwidth of the 32-bit PCI-104 bus lets us serve engineers who need high-speed communications that operate, for example, over wireless IEEE 802.11 connections,” said Colin McCracken, director of product marketing at Ampro Computers. “And the bus works well with a device such as a 32-bit PCMCIA card that requires fast bus traffic.”



PC/104 DESIGN ADAPTS TO THE TIMES

The availability of the PCI-104 bus on a separate connector does not doom new or legacy designs that still need PC/104 boards. Boards that comply with the PC/104-Plus standard offer a 120-pin PCI-104 connector and a traditional PC/104 connector. So, engineers can add PCI-104 and PC/104 I/O boards to a system.



But when SBC vendors use larger, more powerful microprocessors and support chips, they may run out of space and provide only a PCI-104 connector. And, those newer processor usually do not support the old ISA bus. Fear not: Vendors supply “bridge” or “adapter” boards that let engineers mix PC/104, PCI-104, and PC/104-Plus boards in a computer.



EXPRESS BUS HAS ARRIVED

Some engineers want more bandwidth than even the PCI bus provides. So, the PC/104 Consortium has a proposal in hand for an EPIC Express standard that will ease the entry of the PCI Express bus into the PC/104 domain. “Some users need high-speed I/O devices such as Gigabit Ethernet ports and wireless communication devices, which force them to move to a computer that supplies a PCI Express bus,” explained Tom Barnum, a member of the executive management team at Versalogic. “As bandwidth demands rise, we hear from more users interested in the switched fabric of the PCI Express bus.”



Barnum noted the adoption of a PCI Express bus will progress as an evolutionary change as applications require more bandwidth. “Engineers will not pay for higher-priced processors and high-bandwidth communications they do not need.”



The move to embrace an EPIC Express standard will not leave PC/104 behind. Vendors will continue to support many I/O capabilities on the original PC/104 bus. “Until we have an alternative I/O bus, we will continue to see ISA-based applications in embedded equipment,” said Barnum.



For further reading

The PC/104 Consortium Web site provides technical information, bus specifications, and a product-selection guide. www.pc104.org.