October 22, 1998

Computer-board manufacturers are scrambling to get a piece of the action as more customers commit to the CompactPCI platform. Compare the latest and hottest CPU offerings from the industry, and find the one that will speed up your project.

Warren Webb, Technical Editor

The foundation of any new CompactPCI system starts with the selection of a worthy system-slot CPU board. Each CompactPCI bus segment has only one system-slot CPU board, and that board, at a minimum, contains the main processor, provides the clock, manages bus arbitration, and drives the reset line. However, CPU-board manufacturers offer a lot more in the hope of influencing your selection. Higher speed, more memory, lower cost, specialty buses, development support, and custom circuits are just a few of the features that these vendors will use to stimulate your interest. Worldwide, these manufacturers offer more than 60 varieties of Slot 1 cards, each with a custom set of onboard peripherals (Table 1)—just the beginning of a flood of new CompactPCI CPU boards that you can expect soon. Your job, as a CompactPCI user, is to navigate the sea of speeds, options, sizes, prices, and software to select the CPU card that fits your project specification and budget.

Selecting a form factor is one of your first decisions and affects the CPU card, the enclosure, and all the remaining cards. Unlike VME, which almost immediately gravitated to the larger 6U boards, CompactPCI has strong supporters of both the 3U and the 6U sizes. In VME, the 3U size is performance-limited to 16 bits, so users must select the 6U size to have the full 32-bit-wide bus. CompactPCI provides the same bus width of 64 bits in both the 3 and 6U sizes.

"Most of the action up to now has been for 6U boards," says Rob Davidson, strategic account manager at Ziatech. "Initial CompactPCI customers have been datacomm- and telecomm-related customers who design their own peripheral cards. These cards tend to be complex, so the 6U size is more suitable." He says that the 3U size addresses many of the same issues that smaller buses, such as STD, PC/104, and even half-sized PCs, have over the years. Also, Davidson says, "3U fits in the industrial-automation market, where size and ruggedness are more important and the number of rear I/O pins is less important."

The other size parameter is board width, or the number of slots that the card occupies. Offering a three-slot CPU card in a CompactPCI backplane that allows only eight cards sounds like a disaster. However, in practice, manufacturers can adjust the backplane-card spacing to not waste slots. Because the CompactPCI specification does not dictate location, vendors are also free to position the CPU card on the right side of the card cage to eliminate board-width problems. Some vendors list their CPU cards as one slot wide but do not account for the processor heat sink that some operating environments require. In contrast, some two-slot boards may operate in a single slot without a heat sink in efficiently cooled enclosures.

Once you settle on board size, the next critical choice is the operating system. One of the primary operating-system considerations is the amount of real-time activity in the application. If your application must provide a deterministic response to random inputs, you should look at real-time operating systems, such as those from Wind River, Integrated Systems, QNX, and others. The real-time operating system provides the scheduling and prioritization algorithms to support multiple simultaneous tasks.

Another way to handle real-time activity is to include one or more intelligent I/O boards. These boards allow you to offload the real-time portion of your application from the host-computer operating system to an embedded processor dedicated to the task. Intel (www.intel.com) developed the intelligent input/output (I₂O) protocol to standardize the interaction between the host computer and an I/O processor. According to Peter Zackin, vice president of sales at Cyclone Microsystems, many CompactPCI users would rather treat the host computer as a server running Windows NT that communicates using I₂O with distributed intelligent-I/O cards for the real-time problem. This approach eliminates the need to develop NT device drivers, although still requiring real-time I/O programming, and allows application programmers to work in NT. Using Windows NT has the side benefit of expanding the available programmer pool because programmers outside the embedded world can perform these tasks.

Another CPU-board-selection problem is the selection of onboard processor peripherals. CPU-board I/O features range from one RS-232C port to full desktop-equivalent peripheral support (Figure 1). Multiple types of serial communications exist. They include Ethernet, Universal Serial Bus (USB), RS-422/485, Firewire, and several fieldbuses. Mass-storage interfaces include floppy disks, EIDE, SCSI, and flash memory. The range of peripheral interfaces creates a problem for board manufacturers: No matter what combination of peripherals they select, the customer will want something else. The increased use of USB should reduce this problem because this bus removes the interface hardware from the CPU board. The practice of packing every possible peripheral interface onto the CPU board also increases the complexity, number of board layers, assembly time, and, therefore, the price.

Another way to accommodate a range of I/O ports from the CPU board is to include expansion-module sites. Users can plug mezzanine cards directly into the CPU board to allow flexible I/O customization. The most popular type of expansion-module sites are PCI mezzanine cards (PMCs) because they are based on PCI-bus signals. Several vendors also accept smaller PCI module industry pack (PC-MIP) modules and proprietary mezzanine boards. VME boards use both PMCs and PC-MIP mezzanine boards, so a broad selection of functions is available. The exact mix of I/O functions on a CPU board may depend on the target market. As mentioned, the industrial-automation market, in which cost is more important, favors the 3U form factor.

"Cost is a big issue in the CompactPCI automation world," says Kurt Lender, an industrial-automation-application consultant at Radisys. "Customers often ask us to strip functions out of the CPU board to save money. They want only one comm port and one Ethernet port to talk to the main factory computer." Industrial-automation customers also want higher integration. CPU vendors are starting to team with I/O-board designers to combine the CPU with other functions, such as motion control or video-signal processing. Although mezzanine boards provide these multiple functions, industrial-automation customers want even lower costs.

Another feature beginning to appear in CompactPCI is the ability to have more than one CPU board in a system. Previously, CompactPCI CPU boards could not support multiprocessors because of PCI-to-PCI bridge-chip limitations. The C2P2 from General Micro Systems deploys Digital Equipment's (www.dec.com) new 21554 PCI Draw Bridge chip, which connects independent PCI buses while enabling data sharing. Likewise, Ziatech's ZT 5540 accommodates one processor in the system slot and as many as 31 other processors in the same CompactPCI environment.

Too much flexibility?

Both a great feature and a big problem with CompactPCI is its flexibility. A 6U board provides several hundred pinouts through J3, J4, and J5, but the CompactPCI specification leaves these pinouts undefined. I/O-board manufacturers are unsure whether to use the pins or to reserve them for future definitions. Joe Pavlat, president of the PCI Industrial Manufacturers Group (PICMG), recommends that manufacturers use J4 for sub-buses, such as the H.110 computer-telephony bus; J5 for rear-panel I/O; and J3 for anything they like. PICMG is planning to begin a "pin registry," in which manufacturers can register J3, J4, and J5 pin definitions and request a unique IEEE 1101.10 front-panel key that prevents insertion of incompatible boards.

CompactPCI vendors have also found other uses for J4 and J5. Ziatech and Pep Modular Computers increase the number of adapter-card slots by using the J4 and J5 connectors for a second set of independent CompactPCI bus signals. A special backplane routes the second set of bus signals to the corresponding J1/J2 pins in slots 9 through 15. The single CPU card may then accommodate two separate bus segments for a total of 14 adapter cards. The only drawback of this approach is that the CPU-card I/O signals go through J3 instead of the PICMG-recommended J4.

The CPU board you select affects other parts of your system as well. For example, several CPU boards require only 5V and derive any other needed voltages onboard, whereas others need additional power supplies to provide the 3.3, 12, and -12V. Power-supply current requirements range from 1.5A for a 486 to 18A for a dual Pentium II, depending on the processor and speed. A high-power CPU board may also force special cooling techniques in the enclosure.

Long MTBF, short life

Although CPU-board vendors tout MTBF specifications that sometimes exceed 50 years, the actual life will probably be much shorter. Rapidly changing technology is a reality that CPU manufacturers and their customers must face. Although CompactPCI system developers want a design life of five to seven years, the CPU board may need to be updated in less than two years to accommodate higher speeds, new software, and end-of-life components. IC manufacturers look at profits to determine when to stop producing a component. Although manufacturers may target a processor for long-life embedded systems, they stop production if profits fall. CPU-board vendors must then scramble to produce an alternative design.

One problem that you face with CompactPCI computer boards is finding a second source. Most CPU boards are unique, and even if two vendors provide the same processor, memory, and I/O complement, the rear pinouts probably will differ. Rear transition boards solve a portion of this problem. The purpose of rear transition boards is to route all of the I/O boards to the back of the chassis so that cabling does not interfere with easy board replacement. The CPU-board and rear-transition-board combination from two vendors is more likely to be compatible even if the J4/J5 pinouts are different. You can also design your own rear transition board that works with multiple CPU boards to solve the second-source problem.

Most CPU-board manufacturers agree that the primary factor limiting the widespread use of CompactPCI is too few I/O boards. Customers moving from the ISA-bus market are used to hundreds of low-cost I/O boards. To date, ISA vendors have found too little demand to justify the investment necessary to convert many of these cards to Compact-PCI. The VME market also offers many I/O functions, but the design translation is not straightforward because many cards use sub-buses that the higher speed CompactPCI world does not define or even require. VME vendors are also reluctant to enter the CompactPCI market, in which the pricing structure would result in much lower margins. One unusual solution to the dearth of I/O cards is a carrier board from PCI Computer Systems, which allows virtually any PC/104 or PC/104+ card to operate in a 3U CompactPCI system.

CPU-board vendors are experimenting to find the best way to serve their customers. Some vendors choose not to compete directly with the volume-board-production houses. One business formula is to provide a CompactPCI CPU reference design that customers can change as needed.

"Not a single customer has liked the reference design as is," says Terry Furtado, an application consultant at Radisys. "They want something tweaked, some software that is not supported, a funny little driver on the board, or another interface. We don't assume that any of our standard products will go into volume production because the customers will change the design."

Customers also report that I/O boards from different vendors do not necessarily work together. This critical problem results from the necessary evolution of the CompactPCI specification. Boards designed at early stages of the specification may require updates to work with later designs. Older boards are not 100%-compatible with the latest backplanes. To solve these problems, some CPU vendors offer certification programs to verify that their boards operate with the products from other vendors. You avoid many headaches when you are building your first CompactPCI system by selecting boards that manufacturers have tested together.

Despite a number of early development problems, CompactPCI is growing in popularity. CPU-board vendors have taken the lead and are guiding the industry through this early formation period. More than 20 vendors offer an array of CPU boards that include the latest processors and any combination of onboard peripherals your project may need. Recent innovations, including multiprocessing, dual processors, 64-bit processors, and dual buses, will provide you with all the processing power you need. So go ahead, pick a CPU card, and get ready for your next CompactPCI project.

Table 1—Representative Slot 1 computer boards

Acknowledgments

Thanks to Terry Furtado and Kurt Lender of RadiSys, Peter Zackin of Cyclone Microsystems, and Rob Davidson of Ziatech for their assistance.

Warren Webb, Technical Editor

You can reach Technical Editor Warren Webb at 1-619-513-3713, fax 1-619-486-3646, e-mail wwwebb@cts.com.

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