mPCIe Takes on Shrinking Footprints/Growing Challenges
Originally appeared in Embedded Systems Engineering magazine
Keeping solutions rugged enough, and small enough, to support modern mobile applications.
DAQ systems using plug-in slot cards, or even stackable I/O boards, consume a virtual ton of volume and weight, and achieving extended temperature and vibration certification using most COTS hardware is a real headache.
In order to reduce size and improve ruggedness (while still retaining flexibility with regard to changing CPU technologies and requirements) various Computer-On-Module (COM) standards became popular. Instead of DAQ I/O devices being plugged into the CPU’s backplane, the CPU module itself is mated to a single custom-designed carrier board, containing all the application-specific I/O onboard. This carrier board hosts the CPU module as well as the connectors for both the I/O and the standard PC interfaces.
Commercial slot I/O cards (ISA, VESA, PCI, PCI Express) are neither small enough nor rugged enough for many applications. Stackable expansion options, like PC/104 and its faster brethren, are indeed more rugged, but still take up an awful lot of room—and weight—when modern sensor count and mobile applications are considered. These large form factor solutions also encourage a one-size-fits-all approach to I/O design. Thus a wide variety of input and output types, perhaps including voltage, current, frequency, TTL, PWM, quadrature and more, may all be found on one I/O board. Keeping the solution rugged enough, and small enough, to support modern mobile applications requires something different.
Not Just for “Big Gulp” Production Quantities Anymore
The upfront costs associated with a fully custom I/O-equipped carrier board limited widespread adoption of COM technologies to applications with large production quantities. Also, these fully custom COM carrier boards lacked the flexibility necessary to handle changing real world requirements. Suddenly needing a few extra channels of GPIO, or adding a serial interface to a complex instrument over RS422 late in the engineering process, could result in an expensive retooling of the entire carrier module.
Although semi-custom solutions eliminate this retooling when it occurs before volume production begins, some application requirements change after hundreds or even thousands of units have been shipped. Many other applications will never reach the relatively small production volumes necessary to make even a semi-custom carrier board cost effective.
Semi-custom COM solutions came on the market to reduce upfront costs and support smaller production quantities. The ETX-NANO from ACCES I/O, for example, provides a standard COM interface, but rather than a dedicated set of DAQ I/O features hard-wired on the carrier board, a PC/104+ bus is available. Integrators can verify proper operation of their application and software using a COTS COM board coupled with one or more COTS DAQ I/O boards, greatly reducing upfront costs. Only after the selection of I/O is verified—and the application software working—is the carrier board re-spun, integrating the specific I/O channels that were actually used directly onto the carrier board for production scaling.
More Flexibility with Reduced Size and Weight
Adding legacy expansion (like PCI slots, or even PC/104) back to the COM carrier modules would offer the flexibility needed, but these interfaces are large—very large—compared to the modern PCI Express Mini Card (mPCIe).
mPCIe was developed for adding wireless features to notebook computers, isolating the notebooks’ logic board design cycles from the ever-evolving Wi-Fi, Bluetooth and cellular specifications. Unlike its predecessor the PCI Mini Card, mPCIe cards include mounting provisions, preventing any vibration or shock issues common to the older standard. A full size (F1) mPCIe device (Figure 1) is less than 30 x 51 x 6mm in size, and the Industrial and Military DAQ market has pounced on the standard as a rugged-yet-tiny replacement for the larger legacy options.
Selecting mPCIe makes it possible to increase flexibility while reducing size and weight, without sacrificing ruggedness. A COM carrier board or SBC can be designed with all the DAQ I/O and PC interfaces the requirement demands, yet have one, two, even four or more expansion slots available for handling the contingencies of the real world.
Time Savings
Another benefit of the smaller mPCIe standard is a focused approach to I/O expansion: the limited PCB and connector real estate eliminates the unused I/O types and channels common to the one-size-fits-all “multifunction data acquisition system” devices often seen on the larger legacy interfaces.
Performance is assured with the mPCIe’s PCI Express interface, but real flexibility in I/O design is also possible: the mPCIe connector includes a USB 2.0 interface, as well as SMBus and support for future expansion busses. These extra interfaces greatly reduce the time-to-market and upfront costs of bringing new I/O devices to the mPCIe bus, which assures a broad selection of I/O to match application requirements.
Maintenance and repair are also greatly improved over various stackable expansion options: dismantling a PCI-104 stack to replace a failed I/O card is a real hit to system MTTR and usually precludes LRU designation. mPCIe is two simple screws—or even tool-less mounting provisions.
PCIe to mPCIe adapter boards, installed in normal desktop computers, can be used to prototype the I/O before the COM carrier is developed. During the layout of an integrated carrier board it’s possible to have one or more mPCIe boards (which provide the same functionality you’ll be integrating) installed in a single PCI Express slot of a software developer’s PC. Programmers can write and test their code against real hardware months before it would otherwise be available to them.
Additionally, the PCI Express Mini Card specification itself effectively requires each and every mPCIe unit produced be RoHS compliant.
Choosing a carrier board vendor with a broad and deep repository of I/O interfaces already designed on mPCIe, PCI Express, PCI, PCI-104 and USB (to name just a few), helps assure the finished, I/O-integrated, carrier board will use the same schematic and software as the mPCIe prototypes.