Embedded computing designers for demanding digital signal processing (DSP) subsystems in aerospace and defense applications, such as radar, electronic warfare (EW), and signals intelligence (SIGINT), are making broad use of I/O mezzanine modules in the XMC and FMC form factors, yet engineers are worrying that tomorrow's applications-and even some of today's-are finding modern mezzanine modules inadequate.
"We are running out of SerDes and LVDS connections down to the board," explains Marc Couture, director of product management at the Mercury Computer Systems microwaves and digital solutions segment in Salem, N.H. "We're starting to run out of pins." SerDes stands for serializer/deserializer, while LVDS stands for low-voltage differential signaling.
XMC is short for Express Mezzanine Card, while FMC is short for FPGA Mezzanine Card. FPGA refers to programmable processor chips called field-programmable gate arrays. The XMC and FMC are decedents of the once-popular PCI Mezzanine Card (PMC), which has become inadequate for high-throughput DSP applications.
The PMC was designed to provide PCI I/O throughput in a small add-on board that enhanced economy and flexibility by providing standard I/O functionality. VME and PCI board manufacturers have been attracted to implementing I/O with mezzanine boards so they are not forced to redesign their products simply to change out the I/O. Systems designers can evolve their systems to meet growing throughput demands simply by adding different kinds of mezzanine boards.
The XMC version of the PMC adds high-speed switch fabric interconnects for today's VPX, CompactPCI, and other high-throughput single-board computers. The FMC, meanwhile, enables systems architectures to design-in FPGA-based I/O by providing direct links to the FPGA on the mezzanine card.
Until recently, XMC and FMC modules have been adequate for even some of the most demanding DSP applications, but some experts in the embedded computing industry are starting to wish for even more capability than the modern XMC and FMC mezzanine boards can offer.
"In the future there will be a need to refresh the XMC and FMC standards," explains Steve Edwards, chief technology officer for COTS solutions at the Curtiss-Wright Controls Defense Solutions segment in Ashburn, Va. "Applications these are fitting into today, like radar, EW, and SIGINT, are really driving data requirements and bandwidth."
Edwards uses an example of the U.S. Defense Advanced Research Projects Agency (DARPA) Autonomous Real-time Ground Ubiquitous Surveillance-Infrared (ARGUS-IR) program, which is under development at the BAE Systems Electronic Systems segment in Nashua, N.H. For this and other new persistent-surveillance systems, "there are huge amounts of I/O coming into the card."
Other factors in the embedded computing industry also are converging to drive the need for increased bandwidth in XMC and FMC modules. Edwards says the latest generations of analog-to-digital (A/D) and digital-to-analog (D/A) converters require fast serial interconnects.
"We are starting to run out of connectivity," Edwards says. "We have a need for a mezzanine card associated with FPGAs, but which has even more serial connections."
In addition, an anticipated transition to optical computing, which moves data throughout the system with optical fiber or free-space lasers instead of electrical cabling, is expected to ratchet-up the pressure on mezzanine board throughput.
"We keep trying to move more data," says David Pepper, product manager and technologist at GE Intelligent Platforms in Charlottesville, Va. "I'm surprised we haven't had to adopt optics yet, but it will happen. You just won't be able to push the data over copper," says Pepper, who works out of the GE office in Huntsville, Ala.
It isn't only throughput needs that are driving needs to refresh XMC and FMC standards. Some designers worry about the reliability of the standard XMC connector when subjected to heavy levels of shock and vibration found in many aerospace and defense applications, explains Aaron Lindner, engineering manager at Extreme Engineering Solutions Inc. (X-ES) in Middleton, Wis.
Adds Mercury's Couture, "the XMC connector doesn't fare well with multiple re-insertions. You run down the MTBF (mean time between failures) every time you remove one of those things."
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