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Stackable Systems

OneBank: Combining Economy and Performance for Stackable Systems

The longevity and wide use of the PC/104 form factor has been largely a result of its ability to adapt to advances in technology while retaining backward compatibility to earlier modules. The advent of the OneBank connector is an adaptation for board space and cost that fits in smoothly with existing systems.

BY JONATHAN MILLER, DIAMOND SYSTEMS

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The PC/104 form factor, with its unique stacking I/O concept, has been a successful mainstay of small form factor embedded systems for over 20 years. This surprising longevity is due to the powerful combination of benefits that PC/104 provides, including small size, rugged design, use of popular “desktop” or PC bus technologies, and wide range of compatible products from an impressive number of participating vendors worldwide.

PC/104 is, in its essence, the repackaging of PC technology in a physical format conducive to use in industrial and military applications, including both vehicle and stationary systems. The use of PC technology in PC/104 is an advantage whose importance cannot be overstated. The size of the PC market attracted a huge number of semiconductor and software companies, offering embedded systems users access to a universe of low-cost yet high performance features that all worked together easily. This accelerated the adoption of computers into the industrial world and helped usher in a quantum improvement in the quality of life for global civilization.

 Although a 20 year lifespan may impart an impression of “old technology”, PC/104 has in fact evolved over this time to stay current with advances in bus technologies, so that it remains as relevant today as it was in 1991. The term “PC/104” actually refers to a family of form factors and bus connectors, all adhering to the basic 3.55” x 3.775” (90 x 96mm) physical size (Figure 1).

Figure 1
Family of PC/104 form factors from left to right: PC/104, PC/104-Plus, PCI/104-Express, and OneBank. Single-connector configurations may also be used.

The original PC/104 form factor utilized the ISA bus, which was the dominant expansion bus for desktop PCs at the time it was invented. When the PCI bus was introduced and became mainstream a few years later, PC/104 evolved to include it on a second connector, resulting in the PC/104-Plus standard.

PC/104 evolved once again in 2007 to include the latest arrival on the desktop: PCI Express, or PCIe. The ISA connector, having long since been eliminated from the desktop world, was removed, and a new PCI Express connector was inserted in its place. Although the ISA bus is no longer used in desktop and notebook computers, it still enjoys extreme popularity in the embedded world, due to its simplicity and low cost, plus the desire of embedded customers for long term support. Single board computers with the PC/104 ISA bus connector, using the latest embedded processors such as Intel E3800 “Bay Trail”, are still being introduced today.

The PCIe/104 connector, as it is commonly called, consists of a pair of 3-bank surface mount high speed connectors (top and bottom sides of the board). The connectors originally contained a combination of PCIe x1 and x16 lanes. The first bank contains four PCIe x1 links, and the second and third banks contain the PCIe x16 signals.

As embedded technology continued to evolve, it became necessary to provide more options to work with a wide range of peripheral chips and modules. So the original connector pin assignment was renamed Type 1, and a new Type 2 arrangement was offered. Type 2 converted the PCIe x16 lanes into a combination of PCIe x4/x8, LPC, USB 3.0, and SATA interfaces.  Now the PC/104 Consortium could offer system designers a comprehensive solution for board to board connection using the complete range of popular “desktop” bus technologies.

Customers, however, value not just capabilities but also cost and size. On a small board like PC/104, with only 13.4 square inches of total area and about 11 linear inches of available board edge for I/O connectors, the size of bus connectors becomes a real concern as processors, and the SBCs built around them, become ever more integrated. It’s not uncommon for a PC/104 size SBC today to include 2 or 3 different display options, 4 USB ports, 4 serial ports, 1 or 2 Ethernets, SATA, power in, and perhaps some GPIO. All those connectors require space along the board edges for convenient access. With the top and bottom edges mostly occupied with the expansion bus connectors, only about 5.7 linear inches remains on the left and right sides for I/O connectors. Fitting all the required I/O connectors into that small space poses a significant challenge.

In addition to their relatively large size, the PCIe/104 connectors, being sole source and high performance, pose additional challenges to vendors seeking to implement low-cost solutions. PCIe/104 has found great traction in markets which value ruggedness and performance over economy, such as military and transportation. That leaves unaddressed a significant portion of the embedded market still seeking the benefits of PC/104 but not willing to pay the higher price.

The PC/104 consortium responded to these two challenges by implementing a new, smaller and lower cost version of PCIe/104, called OneBank. The goals of the OneBank project were straightforward: Provide a smaller, lower cost way to achieve PCIe/104 stackable expansion, and maintain compatibility with existing PCIe/104 products to protect both vendor and customer investment in existing products and applications.

The pinout of the PCIe/104 connector provided an easy way to accomplish these goals with minimal impact to overall performance. The most popular interfaces, including 4 PCIe x1 links and 2 USB 2.0 links,  were already present on the first section, or “bank”, of the connector, along with 5V power, 3.3V power, and 5V standby power. Consequently, the first bank was sufficient for a large number of embedded applications without requiring any changes. Therefore it was decided to eliminate the two additional banks and retain only the first bank (Figure 2). This resulted in a size reduction of 60% and a cost reduction of similar magnitude.  A newly tooled set of connectors from Samtec made OneBank a reality and ensured full physical compatibility between OneBank and “standard” PCIe/104 boards.

Figure 2
PCIe/104 Type 2 (left) and OneBank (right) connector pinouts. The OneBank connector pinout is identical to the first bank of both Type 1 and Type 2 PCIe/104 connectors.

The smaller OneBank connector yields a valuable 1.7” of board edge back to the board designer, an increase of almost 30%. This extra space can support up to 3 I/O connectors on each side of the PCB, significantly simplifying the design of SBCs with high I/O integration.

An example of a OneBank SBC is the Atlas N2800 board from Diamond Systems (Figure 3). As can be seen, the smaller OneBank connector in the lower left corner of the board makes room for two additional I/O connectors along the bottom edge.

Figure 3
Diamond Systems ATLAS N2800 SBC in OneBank form factor. OneBank connector is in lower left corner.

The OneBank connector inherits all of the design features of the full-size PCIe/104 system. One important feature is lane shifting. Unlike the multi-drop nature of ISA and PCI, PCIe and USB are point to point buses. Each device (I/O board) requires its own dedicated link to the host processor. In a multi-board system, how does a board know which link to use? Lane shifting eliminates the problem. Each board always takes its host interface from the same position on the bus connector. It then shifts the remaining lanes over before passing them on to the other connector on the other side of the board. Figure 4  illustrates how lane shifting works. In this manner, each board is automatically given access to a dedicated host connection without any configuration required by the user.

Figure 4
Lane shifting ensures each board has access to an available PCIe or USB link. Boards mounted above the host processor use the first link, and boards below use the last link. A top/bottom signal on the connector determines which link is selected.

OneBank boards offer two board-to-board spacing heights: The original PC/104 0.6” / 15.24mm and a larger 0.866” / 22mm. The standard 0.6” spacing is more common and provides compact, high-density board stacks. The extended 22mm spacing enables the use of mezzanine sockets on OneBank boards, such as PCIe MiniCards and SATA disk-on-modules. One example of a 22mm board is the E104-MPE-04 PCIe/104 quad minicard carrier from Diamond Systems (Figure 5). Using this carrier, one “slice” of a OneBank stack can contain up to 4 I/O modules, resulting in unprecedented levels of feature density. (One of the sockets is dual-mode and can also support an mSATA flashdisk.)

Figure 5
Diamond Systems E104-MPE-04 OneBank quad Minicard carrier utilizes 22mm stacking height connectors to provide adequate room for Minicards and I/O cables.

As with “standard” PCIe/104, OneBank boards may be implemented either with or without the PCI-104 connector on the upper edge of the board (Figure 6). A OneBank board containing the PCI-104 connector will generally treat it as a “pass-through” connector, meaning the  connector will pass the PCI bus between boards above and below without actually using them. This enables a system designer to include a mix of PCI-104 and OneBank boards in the same stack.

Figure 6
Two OneBank formats are defined: OneBank alone (left) and OneBank with PCI-104 (right).

 OneBank boards may be combined with “standard” PCIe/104 boards in a stack. Figure 7 shows boards of each type mated. When the two types of boards are mixed, the “standard” boards should be installed first (closest to the SBC), and the OneBank boards should be installed last (in the outermost positions of the stack).

Figure 7
OneBank and standard PCIe/104 boards can be combined together in a system. Here a OneBank board is installed above a standard PCIe/104 board.

How many OneBank boards can be used together in a system? The answer may be derived from the signals on the connector: 4 PCIe x1 links + 2 USB 2.0 links means 4 PCIe I/O modules + 2 USB I/O modules may be used, for a total of 7 boards (including the processor), or 8 if you add a DC/DC power supply. That should be plenty for all but the most I/O-intensive applications. If the PCI-104 connector is also present, then an additional 4 PCI-104 I/O modules can be added to the stack.

Although OneBank was just introduced in January 2015, boards are already available from multiple vendors, including Diamond Systems, VersaLogic, Advanced Micro Peripherals, and Sundance Multiprocessor Technology. The range of available products includes SBCs, serial port modules, minicard carriers, FPGA modules, USB ports, and video capture modules. This quick adoption is an indication of strong vendor support that will result in a wide range of products and long lifetime for the OneBank form factor, valuable benefits that users of PC/104 already know well.

The OneBank specification is available for free download at www.pc104.org as part of the newly released PCI/104-Express Specification Version 3.0. There are no royalties or licenses required in connection with the use of the OneBank connector by board designers.

Diamond Systems
Mountain View, CA
(650) 810-2500
www.diamondsystems.com