Two competing stackable PCI Express standards are being unveiled this week at the Embedded Systems Conference in San Jose, Calif. Targeting slightly different slices of the market, the mutually incompatible standards threaten to disrupt next-generation embedded device design much as 4-track/8-track, Beta/VHS, and HD-DVD/Blu-ray rivalries impacted consumer electronics.

Yet, just as 4-track audiotape and Beta videotape players are now distant memories, it’s likely that a couple of years from now one or the other of these new stackable-PCIe approaches will have become the de facto industry standard.

But which one will the market select?

A bit of background

Both stackable-PCIe standards announced at ESC this week are descendants of PC/104, the embedded industry’s first self-stacking PC-compatible architecture, which was formally standardized in 1992 by the PC/104 Consortium. Named for its 104-pin PC expansion bus, PC/104 morphed the IBM PC/AT’s edgecard-style “ISA” (industry standard architecture) bus into a compact, rugged, self-stacking, pin-and-socket equivalent.

PC/104 — a stackable version of the PC/AT bus

With PCI rapidly gaining popularity in the desktop PC market, the PC/104 Consortium in 1996 added a 120-pin pin-and-socket version of PCI to PC/104, resulting in a self-stacking “PC/104-Plus” technology that combines ISA and PCI buses (pictured below).

PC/104-Plus — adding PCI to PC/104

These key technological innovations have enabled a thriving industry consisting of hundreds of vendors of stackable single-board computers (SBCs) and expansion modules. Applications have included medical devices, industrial control, avionics subsystems, portable data loggers, rugged vehicular computers, underwater data acquisition pods, communications systems, and much more.

How PC/104 and PC/104-Plus modules stack

Fast forward to 2008…

With the emergence of PCI Express as a higher-speed PCI alternative for desktops and laptops, it was only a matter of time before the evolving stackable SBC market would require a compact, self-stacking version of PC Express.

The first stackable-PCIe proposal emerged through a mid-2005 collaboration among five companies seeking to add that capability (PDF download) to the 4.5 x 6.5-inch EPIC SBC form-factor. Their proposal, dubbed EPIC Express, defined two bus configurations — a single-bank version with four x1 PCIe lanes, and a three-bank alternative with two additional x4 PCIe lanes. These are illustrated below.

EPIC Express defined two bus options
(Click each image to enlarge)

The right-most illustration above shows an EPIC Express SBC with one three-bank and two one-bank PC/104-sized PCI Express modules installed.

Kissin’ cousins?

Following two years of further development, two descendants of EPIC Express have now been conceived — by two different consortia — and simultaneously will be made public at this week’s Embedded Systems Conference. Here are both groups’ announcements:

• PC/104 Embedded Consortium Approves PCI/104-Express Specification (PDF download)
  

• Embedded PC Leaders Introduce SUMIT Interface Specification (PDF download)

Signs of the common EPIC Express ancestry of these two stackable-PCIe buses are easily seen in the following diagrams, extracted from their respective specifications:

PC/104 Consortium’s PCI/104-Express module form-factor, based on its new PCIe/104 bus (left); SFF-SIG’s Express104 module form-factor, based on its new SUMIT bus (right)
(Click each image to enlarge)

Comparing the specs

While full details of these newly released stackable-PCIe buses are available from each of the two groups, I’ve assembled a brief presentation that summarizes and compares the key features of both specs, and provides some perceived “pros and cons” of each relative to the other. Click the following image to view the comparison (PDF download):

(Click image to view comparison PDF slideshow)

For complete details on the two competing stackable-PCIe buses, please visit the websites of their respective proliferators:

• PC/104 Embedded Consortium
  

• Small Form Factor Special Interest Group

Comments encouraged!

In the above analysis I’ve tried to be as fair as possible to both new stackable-PCIe standards. They both have merit; they both have advantages and disadvantages. Now it’s your turn to weigh in with comments regarding this article, the comparative analysis (PDF slideshow above), or the specifications themselves. Please use the talkback function provided below.