PCIe/104 extends legacy of service in mil/aero

3Interoperability, mechanical ruggedness, and limited Size, Weight, and Power (SWaP) are precursors of every new military/avionics system design, but an evolving military landscape also requires improved bus speeds for applications like HD video transmission. In response, a recent revision to the PCIe/104 specification added support for PCI Express Gen 2 and Gen 3, improving data rates and providing support for new processor solutions. However, the true strengths of PCIe/104 in defense applications are those inherited from the proven legacy of the stack.

Defense systems have a new look, and they are smaller, faster, and cheaper than ever before. On the one hand, the need for a more network-centric defense force has driven down the size of defense electronics, which are still required to provide as much, if not more, horsepower than their larger predecessors. On the other hand, fiscal uncertainty resulted in the Modular Open Systems Approach (MOSA) to defense spending and acquisition to reduce costs and ensure component availability for military/avionics platforms. Together they have created a battleground where Commercial Off-The-Shelf (COTS) technology and Small Form Factor (SFF) hardware intersect.

Due to the tight spaces they will occupy in vetronics, unmanned aircraft, or even man-wearable systems, SFF designs must limit SWaP and provide a high level of mechanical rigidity in an architecture that can accommodate the special requirements of defense applications. Further, they are also required to provide an upgrade path for legacy systems while keeping pace with the latest advances in processor and interconnect technology. To meet these demands, a new revision to the PCIe/104 specification added PCI Express Gen 2 and Gen 3 support in a package that maintains compatibility with previous variants of the PC/104 family. COTS vendors are gearing up to take advantage of this enhanced performance, coupling it with the proven features of the “stack” to solidify PC/104’s place in the modern military.

“Since its rollout in 1991, PC/104 technology has been used for a wide range of land, sea, and air applications. Its small size, flexibility, scalability, long life, and rugged design make it well suited for military environments,” says Bob Burckle, Vice President, WinSystems, Inc. (www.winsystems.com). “A PC/104 card measures 90 mm x 96 mm. This platform size began with the original boards and is maintained with the latest high-performance [PCIe/104] implementation.”

“One of the beauties of the PC/104 form factor is that it has maintained the same dimensions throughout its evolution, from the original PC/104 to today’s latest PCIe/104 architecture (Figure 1),” says Mike Southworth, Vice President of Marketing, Parvus Corporation (www.parvus.com). “The compatibility maintained between PC/104 derivatives helps to increase interoperability. For example, a PC/104-Plus card, which has both ISA and PCI interfaces, can potentially mate with a PCI-104 card by aligning the modules’ PCI bus connectors. In similar fashion, a PCI/104-Express card can potentially mate with a PCIe/104 card. This approach fosters and eases mix-and-match between contemporary and legacy PC/104 module variants, providing the architecture with its well-known “Lego-like” expansion as a result of its pin and header stacking orientation. (Editor’s note: See PC/104 product migration strategies on page 26).

Figure 1: Mechanically, the DuraCOR 80-40 has the same physical robustness and ruggedization levels as earlier Parvus PC/104-derivative systems, but is based on the PCIe/104 architecture to deliver improved databus speed.
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“SBCs based on the PCIe/104 architecture can support the traditional complement of I/O found on desktop PCs, so they typically provide one or two Ethernet interfaces, RS-232/422/485 serial ports, USB ports, some sort of a storage device interface (typically SATA, though a few older cards will support IDE parallel buses), and other interfaces such as a PS/2 port,” Southworth continues. “In addition, these modules will provide some general purpose I/O, as well as video and audio interfaces. To expand their system, the customer may want to stack additional peripheral cards, which are generally tailored to meet application requirements that go beyond those found on a typical PC. For example, to meet the needs of military vehicle or aircraft applications, which often involve a MIL-STD-1553 or ARINC 429 data bus, a couple of channels of 1553 or 429 can easily be added to the system by adding an appropriate I/O card to the stack, or through the use of other types of adapter modules.

“The stacking approach of PC/104 increases ruggedization because every board interfaces to the next board through a very rigid, robust pin-and-socket connection. This connection serves not only as the databus interface, but also as a mechanical interface that keeps the modules connected,” Southworth adds.

Keeping a rugged connection

With shock and vibration being the norm in military systems, a secure PC/104 stack is mission-critical. To achieve mechanical rigidity, PCIe/104 modules retain the PC/104 mounting hole configuration to prevent twisting and flexing of PCBs, while also incorporating robust, locking connectors that ensure signal integrity in harsh environments. Although the PCIe/104 specification does not call for shock and vibe certification, these systems are routinely qualified to MIL-STD-810G, MIL-STD-202G, and other standards required for use in track vehicles, rotorcraft, and fixed-wing aircraft.

“Shock is typically a single event such as dropping a system, sudden acceleration, air turbulence, hitting a pothole in the road, or coupling railroad cars together, which can cause dynamic transients,” Burckle says. “A PC/104 stack is held in place by four standoffs that are inserted into the mounting holes on each board. The purpose is to provide mechanical rigidity to the stack for resistance to shock and vibration. Plus, it prevents separation of the boards when subject to shock. Since the stack is basically a small square, it limits the amount of flex that can be induced into the boards within the stack due to vibration.”

“The corner standoff mounting holes for PC/104 are laid out in a trapezoidal fashion to relieve standing waves and mechanical damage during shock and vibration events common to Military, Aerospace, and Government (MAG) applications,” says J.C. Ramirez, Product Marketing Manager, ADL Embedded Solutions, Inc. (www.adl-usa.com). “As well, where possible, locking connectors are the norm, rather than the exception, to again improve tolerance to high shock and vibration environments.

“From a shock and vibration standpoint, two key attributes to consider are contact wipe length and vertical “slack” of the mated connectors,” Ramirez continues. “Contact wipe is defined as the length of contact surface area between male and female connectors when mated (Table 1). During the development of the PCIe/104 specification, special attention was given to ensure that the QMS/QFS connectors did not fully “seat” when installed at the specified 0.600” board-to-board distance of PC/104. This provides vertical “slack” that helps absorb shock and vibration in rugged applications, thereby minimizing mechanical stress and damage to the connector.”

Table 1: This comparison examines the mechanical characteristics and durability of Samtec QMS/QFS PCIe/104 connectors and TE Connectivity Free Height COM Express connectors. In addition to a mating cycle advantage of roughly 30x, the Samtec connectors feature a rugged, ribbed design for thicker sidewalls.
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A low-power approach to thermal management

As advantageous as their size is for rugged systems, small form factors are as equally challenged with thermal dissipation. In addition to the pressures to fit as much compute power in the smallest package possible, thermal management is complicated in aerospace and defense platforms because active cooling solutions, such as fans, are typically not permitted. To circumvent these challenges, PC/104-derivative systems take several approaches, including attaching processor modules directly to cold plates or internal chassis sidewalls and reducing component count to minimize SWaP.

“Heat is the enemy,” Burckle says. “One of the hallmarks of WinSystems’ PC/104-based SBCs and I/O modules is the use of low power devices. The best way to handle heat is to not generate it. Our goal is to have a power envelope of 10 W or less for the SBC so that handling thermal issues is mitigated. However, one of the ways to dissipate the heat is through heat sinks on the processors (Figure 2). Mounting the processor board at either the top or bottom of the stack can allow heat spreaders to dissipate heat to the chassis. Thermal issues require a thorough system analysis of where heat is generated and its efficient removal to stay within the requisite thermal envelope of the end product.

Figure 2: For many military applications with extended temperature requirements, thermal management is handled through conduction cooling by attaching the processor/chipset directly to a cooling baseplate or internal chassis wall. Pictured here is the ADLQM87PC, a PCIe/104 module based on the 4th generation Intel Core i7 processor, mounted directly to a cooling baseplate. Image courtesy of ADL Embedded Solutions, Inc.
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“The useful board area [of a PC/104-derivative module] is 8640 mm² (13.4 in.²),” Burckle explains. “Subtracting the various connectors yields more or less room depending on the size and number needed for the board. The remainder of the area is for the electronic circuitry that requires the power for operation and subsequent heat generated from it.”

“In regards to voltage, PC/104 derivatives are flexible in that the standard defines support for multiple voltage types, including +5 V, -5 V, +12 V, -12 V, and +3.3 V,” says Southworth. “In practice though, the vast majority of PC/104 cards today run on 5 V input only. Any power conversion requirements are typically handled via DC-DC converters on the card. Standardizing on 5 V is an advantage for system designers because it simplifies and potentially reduces the cost of the power supply design.

“Generally speaking, because PC/104 is a small form factor it does not typically incorporate extremely high power consuming devices,” Southworth adds. “The types of processors used on a server-class SBC that often generate above 75 W are not usually found on PC/104 cards. Typically, PC/104 processors dissipate less than 75 W, and the average power dissipation is usually 15-20 W. Designers of PC/104 SBCs often select low-power Intel architectures such as Atom, Core 2 Duo, and Pentium M-class processors, and lately we are starting to see an increase in the popularity of Intel Core i7-class processors on PC/104 systems.”

A new path forward

Though many of its strengths are inherent to the PC/104 architecture, the advent of PCI Express Gen 2 and Gen 3 support in PCIe/104 promises the bus speeds and processor support to drive next-generation mil/aero applications. With PC/104 products available off-the-shelf from more than 100 vendors worldwide, the stack will remain a proven alternative for aerospace and defense.

“Military and defense thrives on rugged, extended temperature, SWaP-optimized solutions, and PCIe/104 offers all of those,” Ramirez concludes. “Especially with high-performance processors like the Intel Core i7, PCIe/104 is often the smallest form factor available for a given system.”