Choose the right computer form factor

Kristin Allen, VersaLogic Corp. -- Control Engineering, 2/1/2000

Boards, boards, boards. Why use a larger Single Board Computer (EBX for example) instead of a smaller PC/104 CPU? Are there reasons to choose a mature bus such as STD 32 or a more expensive one such as CompactPCI? These questions illustrate the fact that form factor selection is not always simple. Each bus has its own set of advantages for certain types of applications.

Single Board Computer

Single Board Computer (SBC) is the most cost-effective answer for systems that require limited I/O and are not likely to be significantly modified or expanded in the future. Unlike most other form factors, SBCs do not use any type of backplane. The single unified board is simply bolted to the mounting surface and I/O connections are made directly to the board. While SBCs are not as expandable as bus-based systems, they offer excellent performance per dollar. Most SBCs can be customized to some degree with plug-in expansion modules.

SBCs come in a variety of sizes and expansion options. Utilizing an SBC with a standardized layout allows sourcing from multiple vendors. The EBX standard specifies physical factors such as the board size (5.75 x 8 in.), the location and orientation of the PC/104-Plus expansion site (if present), and the placement of the mounting holes.

PC/104 and PC/104-Plus

For seriously space-constrained applications, PC/104 modules are the way to go. An entire system can consist of a single 3.6 x 3.8 in. CPU with on-board video and I/O. To get Ethernet, sound, or analog and digital I/O, simply add another board to the stack. Unlike most other bus standards, PC/104 is expandable, but does not use a motherboard or backplane. The pin and socket stacking design makes each module part of the bus. New PC/104-Plus modules are identical in size and backward compatible with standard PC/104 modules, but use an additional high-speed PCI connection to transfer data. The 32-bit interface with a 132 MB/sec transfer rate is ideal for high-speed requirements such as 100BaseT Ethernet or high-speed video or frame grabbers.

STD

Industrial strength architecture and compact 4.5 x 6.5 in. size make STD bus an ideal solution for real-time control and data acquisition applications. STD 32 Bus (32-bit) maintains backward compatibility with its 8-bit predecessor. Both support nonmultiplexed transfers across the backplane for exceptional performance.

Although a card cage is required to interconnect STD Bus boards, very compact systems can be built using available 3 and 4 slot cages. On the other extreme, the same STD cards can be used in 12 to 24 slot cages for systems that require large amounts of I/O and/or multiple processor power. The advantages of the STD Bus are a unique combination of size, expandability, ruggedness, and fast mean time to repair.

CompactPCI

The bus structure of choice for the growing telecommunications and telephony markets, CompactPCI is the newest of the four form factors. Utilizing a PCI-speed backplane, CompactPCI uses a 5-row x 47 pin backplane with a metallic shroud that acts as a shield.

Most CompactPCI boards have been designed with high performance and/or high reliability in mind. Features such as hot swap and redundant power supplies are usually available for these products. This is less a function of the bus, and more a function of the market. CompactPCI is the most expensive of the 4 buses mentioned here.

Ironically, CompactPCI is not the most compact of the group, measuring 100 x 160 mm for the 3U size and 233 x 160 mm for 6U. The larger card size allows a few more features, such as higher-performance CPUs and larger amounts of RAM. An advantage of the cPCI design is that the card cage also functions as an enclosure. Each board includes a front panel section that fits together with adjoining boards. Faulty boards can be quickly replaced from the front of the unit. The only disadvantage of this scheme is that application I/O connections can not be made to the front of the system. Rear-connected I/O must be defined and carried out by the user.

Kristin Allen, VersaLogic Corp.