The Little Boards That Could
Thanks to advances in integration and updates to the PC/104 standard, the small-format computer boards continue to take on a wide range of industrial and military applications.
In the computer industry, it’s rare for a product to stick around for 25 years. However the small computer boards that follow the PC/104 standard have done just that. First developed in 1987 by a company called Ampro, and standardized by the PC/104 Consortium in 1992, the PC/104 standard for CPU and I/O boards is rapidly approaching its 25th anniversary and is still a viable solution to many embedded computing applications.
A Little Board History
The original PC/104 standard defined a 3.6-in. by 3.8-in. board that employs the personal computer ISA bus for I/O expansion. The physical board format included vertical ISA bus connectors on two opposing sides and four mounting holes. The connectors allow additional boards for I/O and other functions to be stacked on top of the CPU board. By standardizing the position of the mounting holes, stand-offs between the boards keep the stacked boards rigid and keep a constant distance between the CPU board and the additional boards stacked on top of the CPU. That mounting arrangement reduces flexing due to shock and vibration and is thus more reliable than the traditional PC motherboard and vertically inserted cards plugged into a backplane.
A typical PC/104 “stack” might consist of a CPU card, a power-supply board and additional I/O cards from a variety of vendors for serial or parallel interfaces, displays, Ethernet ports, data acquisition subsystems, digital signal processors, or other functions. The CPU boards, predominately based on an x86 processor, could also run a variety of operating systems—Microsoft Windows, Linux, or a real-time operating system. However, due to the relatively low level of integration possible in the early 1990’s, the CPU board was just the CPU, and specialized I/O functions required multiple additional boards.
Standard Upgrades Keep Pace with Performance Demands
For about five years, the 1992 standard allowed designers to create a wide array of CPU and support boards. However as performance demands increased, an update to the standard in 1997 (PC/104-Plus) supplemented the ISA I/O interface with the parallel PCI bus to allow for higher-speed data transfers. The PC/104-Plus CPU boards can use both the PCI and ISA buses, and can thus transmit signals to both ISA and PCI peripheral cards. On PC/104-Plus peripheral boards, the original ISA connector is simply a passive feed-through connector to pass signals to boards higher in the stack. Only the PCI connector is used for communications to other PCI-based boards in the stack. Thus, a PC/104-Plus peripheral card will not work with a PC/104 CPU board. Alternately, a PC/104-Plus CPU board can be used with a PC/104 peripheral board.
A further evolution of that standard removed the ISA bus from the board, thus freeing up some board space, but also making the boards incompatible with previous-generation boards. This PCI version of the standard, PCI-104, was further upgraded by adding a PCI Express interface (PCI/104-express). And, just over a decade later, in 2008, the standard was again updated by eliminating the parallel PCI interface and adding multiple lanes of PCI Express (PCIe) for still higher speed I/O transfers (Figure 1).
A 156-pin surface-mount connector provides the signal interface for the PCIe lanes. Some of the connector pins are also dedicated to additional connectivity buses such as USB, SATA and LPC. There are two configurations defined for the connector: Type 1 and Type 2. The Type 1 configuration provides four x1 PCIe links, two USB 2.0 ports, and one x16 PCIe link. Type 2 also has the four x1 PCIe links and two USB 2.0 ports, but adds two PCIe x4 links, two USB 3.0 ports, two SATA ports, and one LPC port.
Along with the higher speed I/O interfaces, the CPU function also went through many upgrades as the CPU vendors integrated more functionality and increased clock speeds to deliver higher computational throughput and reduced the number of components needed for the basic CPU. The higher level of integration allowed the board vendors to increase the functionality of the CPU board, in some cases reducing system complexity to a single board.
Two larger board format standards, EPIC and EBX (and EPIC express and EBX express), provide designers with additional board space for custom single-board computers. The board sizes, listed in Table 1, include a mounting area for a PC/104 board and space for additional functions. The EPIC board is about 4.5 by 6.5 inches, while the EBX board measures 5.8 by 8 inches (Figure 2). Both EPIC and EBX boards support the stackable PC/104, PC/104-Plus, PCI/104 Express, and PCIe/104 variations and provide headroom for larger heatsinks.
|Name||Primary Purpose||Dimensions||Usable Board Area|
|PC/104||Stackable PC||3.550 x 3.775 inches (90.17 x 95.89 mm)||13.401 in2 (86.46 cm2)|
|EPIC||SBC||4.528 x 6.496 inches (115.00 x 165.00 mm)||29.414 in2 (189.75 cm2)|
|EBX||SBC||5.750 x 8.000 inches (146.05 x 203.20 mm)||46.000 in2 (296.77 cm2)|
Table 1: Board sizes for PC/104, EPIC and EBX
A Look at the PC/104 Ecosystem
The longevity of the PC/104 standard comes with a large ecosystem, with over three dozen board and software suppliers providing a wide range of CPU and I/O support boards, enclosures, operating systems, power supplies, and other functions. One recently released single-board computer is the Fox VL-EPM-19 from VersaLogic (Figure 3). The board contains up to 1 Gbyte of soldered-on DDR DRAM, multiple system interfaces including dual Ethernet ports with network boot capability, four USB ports, four serial ports, a microSD socket for removable flash storage, a SATA interface that supports high-capacity rotating or solid-state drives, and a trio of general purpose timers. The board also supports simultaneous video outputs—both LVDS and analog VGA ports are included.
Designed for low-power applications, the board consumes just 5.5 W thanks to its use of a static-logic x86 processor, the Vortex86DX2 from DMP Electronics Inc. The Vortex system-on-a-chip can run at 933 MHz, and contains a 32kbyte L1 cache, a 256kbyte L2 cache, integrated PCIe interface running at 2.5 GHz, a DDR2 memory controller, ISA, I2C, and SPI interfaces, and an internal peripheral controller. Additional resources on the chip include a DMA controller, interrupt timer/counter, dual Ethernet ports, serial UART with FIFO buffer, multiple USB 2.0 host ports, and an IDE/SATA controller.
Another single-board computer from ADLINK Technology, the CM1-86DX3, targets extremely rugged applications and employs a slightly higher performance member of DMP’s Vortex processor family, the Vortex86DX3. This processor supports up to 2 Gbytes of DDR3L DRAM and can clock at up to 1 GHz. Board features include two Ethernet ports (one 100-Mbit and one 1 Gbit), four RS232/422/485 serial ports, two USB 2.0 host ports, two PS/2 connectors for a keyboard and mouse, and a first-generation SATA port for a hard drive or CD drive (Figure 4). A microSD card slot can be used for solid-state storage or for a bootable flash-based operating system. The system can also be expanded over the board’s PC/104, mini-PCI Express or I2C connectors. Operating systems supported by the card include DOS, various versions of Microsoft Windows and Linux.
The ADLINK and VersaLogic boards are just two examples of how integration has allowed the PC/104 standard to keep current with today’s high-performance embedded system requirements. For a list of all the members and products available, go to the membership and product tabs on the PC/104 consortium website (PC104.org).