June 1998

BY MICHAEL VAUGHN

Pentium II, the newest addition to the P6 family of Intel CPUs, delivers much higher performance while still maintaining binary compatibility with all previous generations of Intel processors. Nevertheless, Pentium II represents a substantial change from existing processor architectures -- including PentiumPro -- a move the company says was necessary to reach the next level of performance and scalability for present and future growth. Pentium II was the solution to previous processors becoming maxed out by the widening gap between CPU clock speed and the ability of the system to get data to the processor. Processor under-utilization caused by restricted bandwidth was magnified in throughput-intensive applications such as digital video and high-end 3D graphics, and as users sought better performance while multitasking and managing high-speed communications.

Pentium II's Dual Independent Bus architecture increases the PC's ability to throughput data to the chip by as much as 300 percent. It uses two separate buses between the processor core and system memory to avoid the processing bottlenecks due to limited bandwidth. In order to accomplish this, Intel was forced to abandon its previous socket architecture in favor of a new, entirely different, package design which introduced the Single Edge Contact (S.E.C.) cartridge concept.

THE OLD SBC FORM FACTOR
As complete as the Pentium II's departure has been from traditional Intel processor architecture, some industrial PC suppliers are basing their Pentium II designs on the familiar passive backplane, plug-in single board computer (SBC) concept. The question is, why continue to develop boards to a form factor that became functionally obsolete with the introduction of Pentium II? It may help to reflect on how the passive backplane single-board computer came about.

In 1994, a group of established industrial computer vendors formed the PCI Industrial Computer Manufacturers Group (PICMG) to adapt existing ISA bus single board computers to the higher performance PCI bus. The resulting PCI/ISA industrial standard specified the logical and mechanical interface between the CPU card and the passive backplane. Instead of using a redesigned edge connector for the CPU card, the PICMG technical committee chose a physical board format and connector based on the old ISA adapter specification and added a PCI connector below and in line with the ISA connector. This allowed sufficient space for 32-bit or 64-bit PCI interfaces. Standard PICMG-type CPU and I/O cards to this day include both ISA and PCI connectors.

PICMG backplanes are typically installed horizontally in a rack-mounted chassis, with the single board CPU and I/O boards plugged into them in a vertical orientation. Single board CPUs and other I/O boards can be removed and replaced very quickly, as opposed to the difficulty of swapping out a PC motherboard. This extremely low MTTR (mean time to repair) is one of the major advantages of the PICMG industrial computer, which has since become the industry standard industrial PC architecture. Unfortunately, what worked nicely for every Intel x86 processor up to and including PentiumPro, does not work for Pentium II.

P2 IS A WHOLE NEW BALL GAME
The game changed radically with the introduction of the Pentium II processor, whose large physical dimensions and mass created special problems other PICMG single board computer designs didn't have to deal with. The new S.E.C. cartridge connector for edge-mounting the Pentium II is not only different from earlier onboard processor sockets, but because of power dissipation specifications of approximately 35W (70W for dual Pentium II CPU boards), the Pentium II requires a massive heatsink and cooling system for proper operation.

Recommended Cooling Alternatives
Data errors, processor failure, and other problems can result if excess heat is not properly eliminated by means of fans, heatsinks, or other cooling devices. To maintain the Pentium II processor's cover temperature and thermal plate temperature at or below 75 degrees celsius for 233 MHz and 266 MHz processors and 70 degrees celsius for 300 MHz, Intel recommends the following cooling options:

With a heatsink only, Intel requires the OEM to provide ducted cooling air to the processor to insure proper air flow. This adds additional chassis weight, reduces room for add-in feature cards, and is far more difficult and costly to mount for vibration and shock tolerance.

Without a heatsink or cooling fan, Intel requires the OEM to design a heatsink and provide the ducting to cool it -- a costly, massive, and delicate approach.

With a combined heatsink and fan, plus mounting braces, Intel requires that FHS (Fan Heatsink) supports be used, whether the processor has an active or passive fan heatsink. This package provides a number of benefits: the best processor warranty, best guarantee of proper operation vis a vis thermal and vibration requirements, takes up far less space than custom ducting, and is readily available to the end user as a replacement/upgrade product if required.

Hardware Must Be Properly Supported
With the fan and heatsink combination, the Pentium II and its associated hardware weigh about 312 grams and occupy 393 cubic centimeters of space. Thus, to use the Pentium II in an industrial or high-availability application, provisions must be made for mounting and supporting the Pentium II with its associated hardware to ensure reliable operation under conditions of shock and vibration, and also to insure survival during normal shipment or transport.

For these reasons, mounting a Pentium II on a CPU card that is plugged vertically into an edge connector, as the PICMG method dictates, is not conducive to long-term reliability. Special bracing is needed to prevent damage due to stress caused by the mass of the processor combined with its center of gravity away from the board. Long-term stress would likely cause damage. And, if the unit were dropped or subjected to shock under operation, the moment of force (its weight multiplied by its distance from the board's surface) created by the Pentium II and its associated hardware could cause fractures in leads and solder joints.

NEW PROCESSOR, OLD BOARD
Most of the advantages attributed to passive backplane SBCs -- very low MTTR, low cost, compatibility, and more -- may be said of the new Pentium II single board computers, with the probable exception of reliability. Reliability shortcomings are largely the result of SBCs built using the PICMG-type form factor. Design problems include but are not limited to:

Processor Orientation: Many SBCs mount the Pentium II and its associated hardware in an unsupported, cantilever fashion at 90 degrees to the vertically mounted plug-in CPU board. Some SBCs also provide connectors on mounting brackets that do not fit the EISA specification. Some SBCs appear to crowd an additional PCI slot.

Application Durability: Some SBCs recommend that users provide their own mounting hardware to support the processor. Other manufacturers who provide no supports do not recommend use in industrial applications.

Cooling: Some SBCs locate the chip set under the heatsink, which blocks air to the DIMMs. Others locate the heatsink under the DIMMs, or provide a custom heatsink/air duct which adds weight to the CPU board and minimizes air flow to components under the heatsink.

Upgradability: Some SBCs offer proprietary fan/heatsinks which require going back to the original manufacturer for upgrades or replacements -- this solution eliminates using standard "boxed" processor components.

CONCLUSION
Many of the problems associated with these approaches can probably be pinned on the headlong rush to be the first one out there with the latest "whiz-bang" CPU board. The bigger issue is the fact that there is still no Pentium II specification in existence, or even being developed, by PICMG, the industrial PC's leading advocate group. PICMG continues to focus its efforts and resources on defining and developing a CompactPCI specification and is not currently addressing issues with other platforms.

Clearly, Pentium II represents a new level of processor performance, and a much more viable highway to the future than other technologies. But along with this comes major new mechanical design challenges for OEMs and others interested in physically implementing it. Suppliers and users alike need to realize that Pentium II is completely unlike any other Intel processor that came before it. For this reason alone, Pentium II requires a correspondingly new approach to mechanical packaging in order to optimize overall performance and reliability of the processor and, by extension, the industrial PC platform itself.

Michael J. Vaughn, director of new product marketing for Industrial Computer Source, brings a varied technical background of industrial instrumentation, communications, data acquisition, and process monitoring to his current position. Industrial Computer Source (San Diego, CA) manufactures and stocks PC-based motherboards, feature card sets, and backplanes for Pentium II, as well as the full gamut of Intel x86-based single-board computers, chassis, I/O, and networking. These product families are targeted for engineers, scientists, and other technical users in the industrial, scientifi,c and telecommunications markets. For more information, visit the company's Web site at www.indcompsrc.com. 

Industrial Computer Source's SBC Solutions

Intel's Pentium II has opened the door to a new generation of high-speed processors. With MMX technology and 512K L2 cache running at half the processor speed, Pentium II offers major system throughput enhancements for CTI, multimedia, and other graphics-intensive applications. Pentium II is not only the next level, it has evolved beyond the support offered by the traditional PICMG single-board computer design specification. In order to maximize Pentium II performance and reliability for CTI and other high-availability applications, a different design approach is needed.

PENTIUM II FOR INDUSTRIAL APPLICATIONS
Industrial Computer Source recently announced its new P2LX Series (patent pending) of Pentium II CPU sets and backplanes, providing full Slot 1 S.E.C. cartridge compatibility and GTL+, ISA-AT, and PCI 2.1 bus support. Extensive laboratory testing was performed to verify hardware and software compatibility and reliability before the product was released to market.

A DIFFERENT APPROACH
Most design solutions implementing Pentium II technology use a cantilever-mounted approach that causes torsioning, PCB stress, and ultimately, board failure. This is caused by the unsupported mounting of the Pentium II's Single Edge Contact (S.E.C.) cartridge and its associated hardware, which weighs more than 11 oz. and occupies about 24 cubic inches in volume (5.5" L x 1.75" D x 2.5" H).

The P2LX is not a typical single board computer (SBC) because the processor is not mounted on a single board. Instead, the S.E.C. cartridge and its voltage regulator module (VRM) are mounted to the backplane where they can be properly supported in the Intel recommended vertical orientation with the retaining hardware. Mounting the S.E.C. cartridge this way also offers better cooling and provides a migration path to later development of dual Pentium II systems that will exceed the performance of traditional SBC designs.

The voltage regulator is incorporated as a separate plug-in module to insure compatibility and upgradability of the system to faster processors as the technology evolves. Since voltage requirements change as speeds increase, the ability to simply pop out an older VRM and upgrade it with a newer module ensures the scalability of the system. Mean time to repair (MTTR) does not suffer either because there is no greater number of active components "soldered down" to the backplane than with any other single-board computer design. The processor, the VRM, and the feature set are all in sockets designed to withstand industrial and commercial applications.

P2LX PRODUCT FEATURES
The P2LX Series has the feature card mounting the 440LX chip set, which was designed for the Pentium II and three initial backplane models (10, 15, and 20 slots) offering a choice of 233 MHz, 266 MHz, or 300 MHz processor speeds. Bundled in a variety of 19" rack-mountable industrial chassis with a variety of power supply options, the basic P2LX model provides a Feature Set Card, Processor, and VRM. Models may also be ordered with optional on-board SVGA video and/or Ultra/Wide SCSI-3 controllers.

So what's next? Additional backplanes will be designed to meet specific user application needs, and future P2LX Series products will be developed and released in parallel to the availability of newer Intel chip sets and processor slot designs. "Slot 2" processors, for example, which are due out in the near future, are expected to pose even greater weight and size challenges than designers are confronted with now. Such indications would suggest that the days of the traditional passive-backplane single board computer are waning fast, at least in applications where system reliability and high availability are of primary concern.

Flush-Mounting The Pentium II

Intel's new Single Edge Contact (S.E.C.) cartridge has presented designers with some interesting challenges. In addition to requiring significant changes to accommodate a new connector and significantly higher profile, the Pentium II's higher speeds and performance necessitated new thermal and electrical designs. How could more performance be eked out of what was already a tightly-engineered layout, while still meeting the mounting and thermal characteristics needed for the ruggedized industrial market?

DESIGN GOALS
The most obvious issue was the height of the Pentium II processor. Installed in the standard Slot 1 connector, it would significantly impact slot availability in existing passive backplane systems by blocking three PCI full-card slots. Retaining the use of these slots was the first goal.

The Pentium II processor specification also lacked the mounting and thermal characteristics needed for the ruggedized industrial market. More secure mounting and the ability to lower the processor temperature without adversely impacting the cost of the board became a second design goal.

MOUNTING OPTIONS
Intel's standard brackets worked well electrically, but did not resolve the backplane slot obstruction problem. Plus, the processor wasn't well supported or stabilized enough by the brackets to withstand vibration levels it would be exposed to in more rugged installations.

Another option was to place the processor module somewhere else in the system -- such as on a backplane. Unfortunately, this option would force the customer into a proprietary configuration requiring backplane replacement in existing systems, or the purchase of specialized backplanes in new installations. In addition to electrical concerns due to longer trace lengths (interference between components), Trenton engineers felt such a design would have the same support and stability problems caused by the brackets.

The third option involved installing the Pentium II processor in the available right angle connector -- but it was still too tall. In order to provide access to all backplane slots (meeting Trenton's first design goal), the processor and memory modules needed to be lower and positioned on the back end of the board.

LOWER PROFILE, SHORTER TRACE LENGTHS, LESS HEAT
Trenton designers liked the right angle connection option, but needed a more compact and rugged method for securing the processor. A concept evolved for a riser card using support brackets screwed through the PCB (Printed Circuit Board: a thin, rectangular plate on which chips and other electronic components are placed) to secure the processor directly to the PCB, providing additional stability. The riser card also created a right angle electrical connection to the Pentium II processor using two Slot 1 connectors to turn all system bus signals. This flush-mounting method was located on the back half of the PCB, allowing use of all backplane slots either with half or full-size boards.

Meticulous care was required to maintain electrical and mechanical characteristics at or above industry and customer requirements. With internal bus speeds predicted to reach 100 MHz and the need to maintain standards while supporting even faster performance bandwidths, trace lengths were closely analyzed and kept to a minimum distance between components, connectors, and the processor.

While addressing the ambient temperature limitations, an idea developed to dissipate heat by positioning a plenum chamber (which controls air flow) over the processor and use a fan to push air across the heatsink. By mounting the fan vertically at the end of the board, it would pull air from outside the card cage instead of recirculating internal air from between the boards. Thus, cooler air would flow through the chamber, pulling more heat from the processor and increasing its thermal operating range. This forward-thinking thermal design could also handle the faster, hotter processors expected later this year without adding extra fan assemblies to the system.

The entire assembly, including the plenum chamber, was only 2.2" high, saving a full-size slot, and making the resulting design the lowest-profile solution offered to date.

The Pentium II S.E.C. cartridge challenges manufacturers to create single board computers that not only maximize every possible bit of incremental speed, but also headroom for faster performance. Design criteria must also insure the stability to hold up in rugged applications and adhere to customers' expectations for standardization and board interchangeability. Trenton's new flush-mounted processor design solves slot obstruction, vibration, and thermal issues while ensuring the electrical integrity for faster bus speeds down the road.

Dwight Justice is the sales manager of Trenton Technology, Inc. Trenton Technology, an electronics engineering and manufacturing company, has built a reputation among OEMs and systems integrators as one of the nation's leading providers of high-quality industrial processor boards and backplanes. For more information, contact the company at 800-875-6031, e-mail them at [email protected], or visit their Web site at www.trentonprocessors.com.