Vehicle computers acquire rugged RAM

The $8,000+ purpose-built vehicle PC from a decade ago has been gradually replaced by generic, affordable “Box PCs.” Under the “hood” one finds a typical 3.5" SBC or sometimes a PC/104 CPU with a stack of I/O cards. The 3.5" SBCs feature either a proprietary bus expansion connector or, occasionally, a PC/104 interface. The trouble lies in the fact that nearly all 3.5" SBCs come with a SODIMM socket; it seems paradoxical to find consumer-grade SODIMM memory modules in commercial and military vehicle applications, even after such diligence was used to qualify thermal solutions, mounting hardware, and either flash-based SSD or shock/vibe-rated rotating hard drives.

The socket is vulnerable to the high-shock and -vibration loads common with vehicle applications. Just a single bit error from an intermittent contact can lead to a minor data discrepancy (best case) or, if the bit is part of a machine instruction, a bad branch offset or even an illegal instruction exception (worst case). The famous “blue screen of death” causes missed surveillance footage or situational awareness displays, for example. A guaranteed gas-tight pin connection is needed in order to achieve the highest reliability.

A new standard emerges

Several PC/104 manufacturers have built proprietary, “closed” RAM modules using rugged connectors during that past 15 years for their own SBCs. What’s needed for mass-market adoption is an “open” standard that is publically downloadable free of charge and endorsed by multiple RAM module manufacturers who are independent of the SBC suppliers.

SFF-SIG answered the call in precisely this manner. The assigned working group decided to focus on DDR3 chips rather than the older DDR2 architecture due to lifecycle and relevance for brand new SBC designs. Virtually all new 32-bit and 64-bit embedded processors come with memory controllers that can talk to DDR3 RAM. The pin assignment closely follows the DDR3 SODIMM pin definition to ease the migration from SODIMM to the new standard. The board outline was created to fit on small SBCs and modules, even MicroTCA (µTCA), which is too narrow to legitimately fit SODIMM sockets. A 240-pin connector from Samtec was chosen for robustness and to support 72-bit data width for high-reliability Error-Correcting Circuitry (ECC). The result is an open standard called eXtreme Rugged Dual-Inline Memory Module (XR-DIMM).

XR-DIMM is a mezzanine memory module standard specifically for the high-reliability embedded systems market, not for consumer markets. The specification defines both unbuffered and registered versions that are analogous to their SODIMM counterparts. The tiny module measures only 38 mm x 67.5 mm. In addition, optional SATA pins pave the way for dual-function RAM + SSD flash modules in the future, allowing SBC and COM manufacturers to reduce the space consumed by connectors.

Confirming robustness

ANSI/VITA 47-2005 (R2007) was chosen as the test methodology to confirm the resilience of the XR-DIMM standard. Rather than reinvent the wheel, the working group members agreed that VITA 47 provides a framework for shock and vibration relevant to commercial and military vehicles alike; The board-to-board mated connector pair plus two mounting holes for screws anchored the module in place to easily pass the test suite.

Upgrading to XR-DIMM

SBC manufacturers who want to target vehicle markets now have a way to differentiate themselves from traditional SODIMM-based competitors – simply place the SBC-side XR-DIMM connector on the board while following the XR-DIMM specification’s design rules. Then the SBC vendor or system integrator can purchase and qualify RAM modules from any of the XR-DIMM manufacturers.

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Figure 1: Swissbit’s XR-DIMM module supports 1, 2, or 4 GB of unbuffered DDR3 ECC RAM in both commercial and industrial (–40 °C to +95 °C) temperatures.
(Click graphic to zoom)

Tiny SBCs like Pico-ITXe, at a mere 72 mm x 100 mm, tend to use soldered RAM, which is certainly rugged, but also fixed, not field-upgradable, or flexible. All larger SBCs are candidates for XR-DIMM memory sockets, especially those targeting fleets, trucks, police vehicles, and military ground vehicles including modern light vehicles. More information about the XR-DIMM specification can be found at www.sff-sig.org.

Small Form Factor Special Interest Group 408-480-7900 info@sff-sig.org