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PC/104 “OneBank” option targets SoC-based SBC designs

Jan 21, 2015 — by Rick Lehrbaum — 1,248 views

[Updated Jan 22] — The PC/104 Consortium updated its stackable PCIe bus spec, adding a “OneBank” bus option with four PCIe lanes, two USB channels, and reduced size and cost.

The venerable PC/104 stackable connector/mezzanine form-factor has gone through half a dozen major updates during its 24-year history. This time, the advancement takes the form of the addition of a significantly more compact OneBank bus connector option, added as part of rev 3.0 of the PC/104 Consortium’s “PCI/104-Express and PCIe/104 Specification.”

We first encountered this shrunken PCIe/104 bus option in a preliminary, pre-ratified form as part of our coverage of Diamond Systems’s Linux-friendly Atlas SBC one year ago. Several months later, the new bus option popped up again, accompanied by the “OneBank” moniker, in VersaLogic’s announcement of its similarly Linux-ready Bengal SBC.

OneBank-expandable SBCs: Diamond Systems Atlas (top view) and VersaLogic Bengal (bottom view)
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According to the PC/104 Consortium’s v3.0 spec announcement, various SBCs and expansion modules using PCIe/104 OneBank expansion are either already shipping or under development by vendors including Sundance, VersaLogic, and Diamond Systems. (See farther below for more module photos.)

PCIe/104 original and OneBank connectors
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The OneBank connector, supplied by Samtec, subtracts two of the three banks of the PCI Express connector, resulting in a 52-pin connector instead of the 156-pin original one. The design provides an additional 10 percent of the original board space, enabling more I/O connectors or a smaller board, says the consortium. To its credit, perhaps, the PC/104 Consortium does not use the term “Internet of Things,” but we think it’s fair to say this new OneBank option is much better suited for building small-form-factor IoT gizmos. OneBank is also claimed to lower the bus connector cost to about $7 in 5k quantities — or $14, if both the top and bottom bus connectors are required for bus passthrough.

Comparison of “104 form-factor” stackable PCIe and PCI options
(click image to enlarge)

The new OneBank connector is designed to plug into the bottom of the leftmost bank (Bank 1) of the PCIe/104 connector. As a result, the connector maintains compatibility with PCI/104-Express and PCIe/104 modules for stacking and data transfer. Meanwhile, a PC/104-sized OneBank board can plug into any existing PC/104 board with a three-bank PCIe connector.

PCIe/104 OneBank and full-width buses can stack together
(click image to enlarge)

OneBank’s reduction in pins by two thirds reflects the evolution of modern system-on-chips, which often integrate graphics and DSP functionality. As a result, modern SoCs make redundant much of the data transfer functionality available via the maxed-out PCIe/104 “Type 1” option’s PCIe x16 channel. For SBC designs based on such SoCs, OneBank can eliminate the unnecessary PCIe x16 channel, along with several other signals, thereby freeing up board space, BOM costs, other resources consumed by those pins, while still maintaining four PCIe Gen 3 lanes and other core functionality.

Comparison of PCIe/104 Type 1, Type 2, and OneBank bus signals
(click image to enlarge)

As indicated in the table above, a single board computer equipped with a OneBank expansion bus supplies four PCIe x1 lanes (configurable up to x4), two USB 2.0 ports, ATX power and control signals (+5V standby, PS on, Power Good), +5V and +3.3V DC power, and Ground. Meanwhile, performance should meet PCIe Gen 3 levels of 8GHz and 985Mbytes/s for each lane, says the PC/104 Consortium.

OneBank-interfaced expansion modules from Sundance (Xilinx FPGA carrier) and Diamond Systems (octal serial I/O)
(click images to enlarge)


OneBank expansion modules, from Advanced Micro Peripherals: video framegrabber (left), quad USB (middle), PCI/PCIe bus bridge (right)
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OneBank expansion carrier for four mini-PCIe cards
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The OneBank standard can also be used with SBCs or carrier boards outside the realm of the PC/104 Consortium’s own form-factor standards. Any baseboard or SBC, whether custom or standardized, could add a OneBank connector and then stack OneBank-equipped mezzanine boards, says the consortium. Options might include CompactPCI, VME, OpenVPX, and more. The PC/104 Consortium goes on to say that “OneBank-equipped boards are not limited to the PC/104 form-factor so the possibilities are huge both within and beyond the world of PC/104.” The photo below shows the pairing of the OneBank-interfaced Sundance FPGA module with Fastwel’s CPC805 EPIC-Express SBC.

OneBank FPGA module installed on a three-bank EPIC-Express SBC
(click image to enlarge)

Further information

The PCI/104-Express and PCIe/104 Specification v3.0 (with OneBank) is available now from the PC/104 Consortium. OneBank SBCs and I/O modules from companies including Sundance, VersaLogic, Diamond Systems, and possibly others will be showcased at the RTECC show on Jan. 22, at the Santa Clara Convention Center in Santa Clara, Calif. In addition, the PC/104 Consortium will be presenting an RTECC session titled, “PCI/104-Express Version 3.0 with OneBank is Here!” More information on OneBank may be found at the PC/104 Consortium website.


Full disclosure: LinuxGizmos founder Rick Lehrbaum helped create the PC/104 form-factor as CTO of Ampro Computers, and chaired the PC/104 Consortium for many years, but is no longer associated with the PC/104 Consortium. A history of PC/104 standards may be found in’s 20th Anniversary PC/104 retrospective, posted in Sept. 2012.


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