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New System Specifications

Rugged Memory Spec Raises the Bar for Rugged Modular Computing

Securing memory modules against shock and vibration on small modules has been a challenge often involving epoxy and adhesives. Now a new standard specifies a proven method to secure memory modules to COM modules.

MARKUS FRIESE, LIPPERT EMBEDDED COMPUTERS

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The popularity of COM Express has positioned the modular computing standard for penetration into more demanding markets such as intelligent transportation, military ground vehicles, aircraft and avionics, outdoor mobile communications, imaging, facial recognition as well as shipboard and marine computing. Modules are widely available with mid-range to high-performance embedded processors and chipsets.

For a variety of reasons including costs and inventory management, the vast majority of COM Express modules are designed with consumer SODIMM RAM sockets. As far as rugged application suitability is concerned, the use of SODIMMs has been a drawback compared to the many low-end to mid-range PC/104 SBCs that utilize soldered memory. The ruggedness of RAM chips soldered directly on a host SBC or computer-on-module (COM) is undeniable, as established by numerous shock and vibration test reports and many hundreds of rugged application deployments over the years.

However, system OEMs in these markets who want access to the latest dual core processors have few choices other than ad hoc ways of retaining commercial-grade RAM sticks into their sockets.

Brute Force

The most popular practice for securing SODIMMs against high shock and vibration is applying epoxy everywhere the module touches its socket. Compounds such as RTV silicone adhesives and sealants and epoxy encapsulants are well established for potting components and attaching cables to header connectors, so holding card-edge boards into sockets with epoxy seems reasonable. Breaking the epoxy seal is not an issue as long as the SODIMM manufacturer is lenient when accepting warranty returns.

Other methods in use range from clips to brackets to clamps to screws to thermally attaching to the same heat spreader that cools the dual-core CPU. Of course, exposing system memory to tens of watts of heat doesn’t improve the long-term reliability of the RAM. Concerned about contaminants and corrosion 10-20 years after deployment, some system OEMs have even asked their embedded computer suppliers for evidence that the gold plating won’t wear out due to micro-etching of gold fingers against socket pins.

The Pursuit of Ruggedness

System manufacturers targeting harsh environments with their high-performance applications should have the freedom to select COM Express without fancy fastening, fixturing and finger-crossing. Enter RS-DIMM—a new multi-vendor standard from the Small Form Factor Special Interest Group (SFF-SIG). The primary goal in creating this standard was to address the need for reliable memory by using a proven board-to-board connector pair methodology with the additional support of mounting holes (Figure 1).

Figure 1
RS-DIMM Module from Swissbit.

As early as 10 years ago, the need for rugged RAM drove some PC/104 CPU manufacturers to design memory modules when large processors and chipsets prevented soldered RAM chips from fitting on board. But these were sole-sourced proprietary RAM modules. RS-DIMM solves this problem two ways—with multiple manufacturers out of the gate, and with a neutral and impartial standards organization responsible for the long-term maintenance, support and revision control of the governing specification. Swissbit is already in production with the module, and Virtium is prepared to begin production shortly. Having two or more sources is usually important to reduce supply risks.

The RS-DIMM Specification defines a small 67.5 mm x 38 mm module that stacks 7.36 mm above the CPU board. Ruggedness is achieved by using a 240-pin Samtec BTH/BSH connector pair on the memory module and the CPU board, along with two mounting holes. It fits better on a number of small form factor CPU boards than the wider 72 mm SODIMM modules. Finally, the pin definition for RS-DIMM closely aligns with the SODIMM pin definition, making it straightforward to migrate an existing CPU board to RS-DIMM (Figure 2).

Figure 2
RS-DMM module diagram.

Aside from signal integrity, the salient feature of the Samtec connector pair is the gas-tight connection resulting from adequate vertical wiping during installation. Gold-plated card edge fingers snapping into a consumer socket don’t come close to the same level of air-free contact assurance.

Performance with Data Integrity

RS-DIMM uses DDR3 technology and is specified for both unbuffered and registered implementations. DDR3 is the state-of-the-art high-performance RAM technology for two of Intel’s three embedded roadmaps: the high-end Core family (Core i7), and now the mid-range roadmap featuring Atom D425 and D525 “Luna Pier Refresh” dual core Atom processors. Last year, the mid-range N270, N450, D410 and D510 processors still featured DDR2 memory, but D425 and D525 processors are replacing them for new designs. The only remaining Intel embedded roadmap using DDR2 memory is the Atom Z-series and its successor Atom E-series processors. These designs are typically provided in small form factor modules with soldered RAM, such as the tiny CoreExpress modules. Consequently, the need for a rugged DDR2 module is rapidly vanishing from the market.

Memory sizes up to 4 Gbyte are currently supported on RS-DIMM, with optional ECC (error correction circuitry) supported using either 9-chip or 18-chip designs. ECC provides an extra level of data protection for high-reliability applications. Intel has integrated ECC support into Core i7 memory controllers, but not into Atom memory controllers.

It’s not hard to see the advantages of a mated connector pair over consumer card-edge-socket schemes. But the proper engineering verification approach is to select a test methodology along with limits that are meaningful and suitably representative of the rugged applications. For 15 years, the stackable processor and I/O community has used 12 Grms vibration and 50 Grms shock in both directions along each axis. This is often referred to as a 6-axis test. ANSI/VITA 47-2005 (R2007) provides the necessary framework.

According to the VITA organization, this specification defines environmental, design and construction, safety and quality requirements for commercial-off-the-shelf (COTS) plug-in units (cards, modules, etc.) intended for mobile applications. COTS plug-in units are widely used in commercial, military, ground, aerospace and mobile applications. Certification of COTS plug-in units, by supplying vendors, to this standard will facilitate the cost-effective integration of these items in larger systems.

VITA 47 V3 and OS2 Tests were performed to establish the ruggedness of the memory module. To ensure the integrity of the tests, an EPIC form factor SBC was used for the testing to eliminate any chance of COM Express connectors and mounting affecting the test results. During the V3 tests, three boards passed the random vibration profiles. The RS-DIMM modules were tested over vibration frequencies from 5 to 2000 Hertz.

In the OS2 half sine wave shock tests, the DVI display adapter cover (shroud) detached from one of the carrier boards, but that unit still functioned properly. Both boards passed this six-axis shock testing (three orthogonal directions, positive and negative excursions). In addition, four shocks were performed in the form of a bench drop shock test, and the units under test (UUT) passed.

A Delivery Vehicle

The first commercially available RS-DIMM module was shown by Swissbit (Figure 1) during the Embedded World 2011 show in Germany. The first COM Express module to support this new RAM standard with ECC was demonstrated during the same show (Figure 3).

Figure 1
RS-DIMM Module from Swissbit.

Figure 3
The Toucan-QM57 supports both soldered and RS-DIMM memory (right edge).

Lippert’s Toucan-QM57 is a high-end COM Express revision 2.0 Type 2 module with an Intel Core i7 processor. The module is specifically built for applications exposed to rugged environments. This is underscored by the integration of the highly rugged RS-DIMM memory module. The memory module is fastened by screws. The raw computation performance of the processor and DDR3 RAM lends itself to image processing, video encoding, communications and other demanding tasks. The COM Express module offers 4 Gbytes of soldered RAM and another 4 Gbytes of RAM by way of RS-DIMM card, which optimizes board space and ruggedness. 

With the 1.06 GHz dual-core Core i7-620UE processor, the module operates over -40° to +85°C for military, energy and other outdoor environments. The 2.53 GHz dual-core Core i7-610E processor model is rated to +60°C for avionics, transportation, marine, imaging, communications and some automation applications. For these applications, additional embedded features add to the rugged reliability. Lippert Enhanced Management Technology (LEMT) provides condition monitoring and supervision capabilities with a convenient API for popular operating systems.

A Graceful Landing

Three off-the-shelf production CPU boards on their heat-spreaders were subjected to stringent VITA 47 testing. Apart from a loose screw and a DVI connector cover, which were considered to be non-critical, the RS-DIMM modules all performed brilliantly.

RS-DIMM has been proven to withstand extreme shock up to 50 Grms and vibration up to 12 Grms along three orthogonal axes. These amplitudes are well established in these target markets over the years, as is the VITA 47 test methodology. Due to the use of board-to-board mated connectors rather than commercial-grade card edge gold fingers, RS-DIMM allows even COM Express modules with high-performance Intel Core i7 processors to be ruggedized for the stringent requirements of outdoor fixed and mobile applications. Standards from three trade groups—VITA, PICMG and SFF-SIG—were tapped to generate this best-in-class modular solution. RS-DIMM allows modular computing to attain a great level of rugged reliability without epoxy, brackets, clips or other support. 

LiPPERT Embedded Computers
Mannheim, Germany.
+49 621 4 32 14-0.
[www.lippertembedded.com].

 

Swissbit
Bronschhofen, Switzerland.
+41.71.913.03.03.
[www.swissbit.com].

 

Virtium Technology
Santa Margarita, CA
(949) 888-2444.
[www.virtium.com].