Ruggedizing RAM for industrial systems
When it comes to rugged computing, the ANSI/VITA 47-2005 (R2007) specification has earned its stripes as a guide for validation testing and product development. We like our backplane connectors, circuit boards, and chassis mounting structures to be reliable over shock, vibration, and temperature loads. Too often, however, the Single-Board Computers (SBCs) inside are using consumer DIMM or SODIMM memory sticks. The Small Form Factor Special Interest Group (SFF-SIG) addresses this problem with XR-DIMM.
“Industrial systems” is a broad category that describes computing devices in many form factors deployed in many environments. Such environments include extremes of temperature, shock, vibration, humidity, dust, and so on – not a place for your average desktop or notebook computer, nor for the average computer RAM. Simply calling a motherboard “industrial” doesn’t fix the RAM connector vulnerability.
Low-cost motherboards feature vertical DIMM slots or horizontal SODIMM card-edge sockets that were developed for computers that sit on desks and in datacenters, not for industrial environments where temperatures vary over much wider ranges. Rapid temperature changes create conditions known as “thermal shock.” If the connector pins don’t wipe the gold-plated card-edge fingers well enough, tiny air pockets can form between the metal surfaces. This can cause code and data bit errors due to intermittent contacts or open circuits. The risk is worse depending on humidity and dust in the environment, and even shocks caused by forklifts and machines.
Reconnecting the boards
By contrast, board-to-board mated connectors are designed to achieve high reliability in these types of environments. Board-to-board mated pairs have substantial connector wiping during insertion, and also have strong retention forces. The SFF-SIG used these benefits to define a truly rugged-by-design module standard called XR-DIMM. Ruggedness is achieved by using a 240-pin Samtec BTH/BSH connector pair on the memory module and CPU board, along with two mounting holes (Figure 1). The module can be secured with two screws to better resist shock and vibration, in sharp contrast to the socket “wings” that hold a SODIMM in place.
The name “XR-DIMM” stands for “eXtreme Rugged Dual In-line Memory Module.” The XR-DIMM specification defines both unbuffered and registered versions that are analogous to their SODIMM counterparts while also being tested for ruggedness according to the ANSI/VITA 47-2005 (R2007) specification.
The right sizing
XR-DIMM is a small 67.5 mm x 38 mm module that stacks 7.36 mm above the CPU board. There is enough space underneath for tiny passive components on the surface of the CPU board, reducing the overall volume occupied by the electronics. XR-DIMM uses DDR3 technology, which is mainstream in the industrial market. Memory sizes up to 8 GB, with optional Error-Correction Circuitry (ECC), are supported using either 9-chip or 18-chip designs. Using a module offers greater capacity and flexibility compared to soldering RAM directly to the CPU board. XR-DIMM is narrower than a SODIMM, allowing it to fit on most small form factor CPU boards, even on processor AdvancedMCs (AMCs).
The pin definition for XR-DIMM closely aligns with the SODIMM pin definition, making it easy to convert an existing SODIMM-based design to a rugged XR-DIMM-based design. SMART Modular, Swissbit, and Virtium are building XR-DIMM modules, some of which have ECC memory for the best reliability in industrial systems.
An eye to the future
Besides RAM signals, the XR-DIMM pin definition also includes a SATA interface to enable the development of dual-function modules containing both DDR3 and flash memory for a Solid-State Disk (SSD) implementation in the future. For forward-looking designers, it’s even possible to create a high-value power-fail solution that copies from RAM directly to the SSD on the same module, since shutting down a large Operating System (OS) takes many seconds.
The SFF-SIG continues to recommend the use of true embedded building blocks in the design of industrial systems, and offers a full family of CPU and memory module specifications for board designers. The XR-DIMM specification is freely available on the SFF-SIG website (www.sff-sig.org) and can be downloaded free of charge and without licensing or registration.
Small Form Factor Special Interest Group 408-480-7900