Number 7 in an ongoing series of articles by employees here at ACCES

Originally appeared in

Pico-I/O puts the small back into small form factor I/O

Integrating next-generation embedded I/O into a small form factor board requires more than just adding a modern interface. Stephen describes the origins of the Pico-I/O standard and explains how its I/O-stacking features enhance space-constrained embedded applications

In early 2003 I wrote about the future of PC/104 in the embedded marketplace. At the time, many in the industry speculated that the mostly ISA-based stacking standard was dead. Despite this cynicism, I argued that PC/104 was the last bastion of the ISA interface and that it had many more years of life left. A new interface would come eventually, but the PC/104 form factor still served too many existing rugged, defense, and mobile applications to become obsolete.

The reality today is that embedded I/O involves more than just replacing an existing form factor with a modern interface. Although several new embedded SBC small form factors have come into existence since PC/104’s introduction, small, rugged I/O form factors have not proliferated correspondingly. The ISA and PCI parallel bus interfaces are the primary reasons and limitations for shrinking a stacking form factor smaller than the PC/104 footprint. It is not by accident that PC/104 connectors for PCI and ISA take up almost the entire width of the board. With the advent of lower-pin-count and high-speed serial I/O interfaces such as USB and PCI Express, the dynamics of embedded I/O boards are changing, as demonstrated by the new Pico-I/O form factor.


In 2007 the PC/104 Consortium began focusing on the need to replace the ISA interface with a PCI Express connector of equal size but greater capacity. Meanwhile, that same year, the SFF-SIG formed to meet the needs of new embedded form factors apart from the PC/104 standard. The group’s first objective was to create a smaller, more versatile stacking computer I/O interconnect that was independent of any particular SBC or I/O board form factor. Using a variety of low-pin-count interfaces, SFF-SIG developed the Stackable Unified Module Interconnect Technology (SUMIT) standard, which specifies a standard stacking connector and pin-out regardless of the board form factor. Each SUMIT connector is about one-third the size of PC/104 connectors.

The SUMIT A connector is populated with all the interfaces needed for average I/O performance, including USB, LPC, SMBus or I2C, SPI/Microwire channels, and a single x1 PCI Express lane for ExpressCard support or higher-performance I/O cards. SUMIT A can effectively replace most of the typical I/O applications for PC/104.

The SUMIT B connector is a higher-performance interface with two configurations specified depending on the PCI Express chipset support available on the SBC. The original configuration supports another x1 PCI Express lane and a PCI Express x4 lane. The second configuration supports a total of five PCI Express x1 lanes. SUMIT B can effectively replace applications requiring performance equal to or greater than PCI-104.

When used together, SUMIT A and B can provide up to six x1 PCI Express lanes in the stack. Like PC/104, SUMIT can support either or both of the SUMIT interface connectors. Unlike PC/104, SUMIT puts both its connectors on the same side of the board. SUMIT also stacks in only one direction using all the different interfaces, keeping SBC costs down and encouraging passive cooling for high-performance processors. Processor boards can use taller connectors and standoffs to create additional space for CPU and chipset cooling under an I/O stack. Alternatively, CPUs can be placed on the opposite side of the I/O stacking connectors, which would allow passive metal cooling plates to be mounted against an enclosure.


To integrate existing PC/104 I/O product lines with the new SUMIT interface, SFF-SIG developed the Industry Standard Module (ISM) with measurements equivalent to PC/104. When using the SUMIT interface, the resulting board is called SUMIT-ISM (see Figure 1). This standard fulfills the need to support legacy PC/104 interfaces and coexist with SUMIT’s high-speed, low-pin-count interfaces in the same stack.

Figure 1: A SUMIT-ISM I/O LAN board from VersaLogic offers a SUMIT AB Type 1 configuration.

In the SUMIT-ISM Legacy Type 1 configuration, the SUMIT connectors are located in the same area as where the PCI-104 connectors are placed on a PC/104-Plus module. The PC/104 ISA connector is in its typical PC/104 location and has the exact same characteristics as defined in the PC/104 specification. Standard PC/104 mounting holes are used. In the SUMIT-ISM Legacy Type 2 configuration, the SUMIT connectors are located in the same area as the PC/104 ISA connector, and the PCI-104 connector is located opposite the SUMIT connectors.

Because the mounting holes on the PC/104 standard are slightly asynchronous on one side, SUMIT-ISM employs slotted or oblong holes so that a legacy I/O board can be turned around and used on the opposite side of the original mounting location.


While the initial purpose for developing the SUMIT interface was to form the SUMIT-ISM standard, another significant outcome resulted from its creation: the ability to establish a new, small, stacking I/O form factor called Pico-I/O, which the SFF-SIG released last year.


With 60 mm x 72 mm or 4,320 mm2 board area, Pico-I/O has one-half the approximate board real estate of the ISM or PC/104 size of approximately 90 mm x 96 mm or 8,640 mm2. Given that a SUMIT connector is about one-third the length of a PC/104 connector, sufficient space is available for less dense I/O applications.

The most popular interfaces on Pico-I/O are LPC, PCI Express, and USB. LPC offers a low-pin-count and low-cost interface that can replace many legacy ISA functions by using FPGAs to provide limited parallel bus functionality. Multiple LPC devices can be used on the same interface. The x1 PCI Express lanes provide high-performance interfaces from one or two boards with options for up to six x1 lanes on both SUMIT A and B. For example, a x4 lane enables a high-performance video card without the massive power use of x16 graphic cards, which make little sense in embedded applications that need lower power and less heat generation. The four USB ports allow up to four USB 2.0-based boards to be stacked. ACCES I/O Products has introduced a line of Pico-I/O boards supporting USB on the SUMIT A connector, as shown in Figure 2. For non-SUMIT-based embedded SBCs, ACCES also provides a line of USB Pico-I/O boards available with a USB micro header and Micro B connector.

Figure 2: ACCES I/O Products’ stacking USB Pico-I/O board supports the SUMIT A configuration.


Electrically, Pico-I/O can support more than 10 I/O boards in a stack. The real benefit, however, is the capability of adding I/O cards from a variety of computer interfaces for any given embedded application.

Mechanically, like PC/104, Pico-I/O uses four standoffs to mount boards securely in the stack for shock and vibration purposes. Unlike PC/104, Pico-I/O uses four symmetrical holes, allowing any SBC holes with the same corner placement to be shared (see Figure 3). Pico-ITXe’s width and Pico-I/O’s length measure at 72 mm, and both standards have the same corner holes. An SBC can share as many as two mounting holes, which would only require using two additional holes to mount the smaller Pico-I/O. This frees up space for extra features often required in many of today’s dense embedded SBC designs.

Figure 3: Pico-ITXe can be configured without shared Pico-I/O mounting holes (left) or with up to two shared Pico-I/O mounting holes (right).


With its rugged and space-saving features, Pico-I/O is well-positioned to help embedded I/O catch up with continuously shrinking embedded computers. As the ecosystem for this standard grows, innovative data acquisition and control systems utilizing Pico-I/O will foster many new applications serving the embedded market.

For example, wind turbine control systems need small, low-power SBCs to collect data and monitor equipment status. Each electricity-producing turbine in a field of turbines is networked and connected to a remote central monitoring unit. This type of application is a perfect fit for Pico-I/O. If the network goes down, Pico-I/O modules mounted on each turbine can continue to collect critical data and/or make decisions to maintain or shut down the unit.

Another application example involves underwater weather buoys that collect environmental data throughout the ocean. Parameters such as air and water temperature, salinity, acidity, wind speed, and wave height must be measured and acquired. Created decades ago, these large buoys are now being replaced with smaller, less costly units that require embedded computers and I/O mounted in cylinders where space and power are at a premium. Similarly, UAVs manufactured for the military are demanding smaller systems to fit the required electronics within a limited plane body diameter. Whereas a PC/104 stack might not fit these narrow space constraints, a Pico-ITXe CPU board with a Pico-I/O module can be easily slotted into such tight locations.

Other Pico-I/O application examples include wearable computers for the military and portable medical instruments for field service. The increasing development of robots and androids also calls for a small I/O standard to provide new functions in both the civilian and military markets.


Pico-I/O has a place in the potentially restored post-industrial economy. Improved productivity caused by ever-increasing technological development and lower-power computer processors will create applications for artificial intelligence that will service the existing and future needs of society. Whether it is saving lives with portable medical equipment, monitoring and controlling outdoor green industries, or providing autonomous vehicles for the military, a rugged, standardized, compact I/O board has its niche.

Acquisition, Control, Communication: Engineering and Systems
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