Rethinking the Bus Using USB

Categories: Press Releases

Originally appeared in

With USB so prevalent and PC/104 so popular, why not put the two together? Combining the form factor of PC/104 with the fast interconnect capability of USB creates the new USB/104 form factor.

Parallel buses dominated the early phases of embedded computing. Several form factors, some based on backplanes and some based on stacking modules, developed around those parallel buses, standards such as PC/104, VME, and CompactPCI. Each of these form factors has its own set of connectors and mechanical standards requiring CPU cards and I/O cards with specifically compatible designs.

Many applications that use these parallel bus-based form factors are often limited by the space, number, and type of I/O slots available in any given computer system. Equipment designers continue to struggle with functionality versus size versus available slots or stacking locations.

Serial buses present a unique opportunity to rethink the way form factors are used and how boards and systems interconnect. USB in particular has emerged as an interconnect standard that has enjoyed rapid adoption in the consumer and embedded spaces during the past few years.

Realizing the advantage USB brings to the industrial marketplace, industrial and embedded I/O vendors have released a multitude of industrial I/O boards packaged into various enclosure types. No longer does a particular CPU card require I/O to be built to its bus form factor. USB I/O systems are quickly realized by merely connecting the I/O by wire to any computer with an available USB port.


The high performance of USB is getting faster with the advent of USB 3.0. With the goal of reducing space in embedded applications, designers chose the widely used PC/104 form factor as a starting reference point for this new USB I/O concept. PC/104’s small size, rugged design, hundreds of boards, and dozens of manufacturers made this an easy decision. The objective was to design a versatile OEM USB form factor that can be used inside and outside its own enclosure.

In developing the new USB/104 form factor (shown at the top of this page), designers integrated the same PCB size and predrilled mounting holes as PC/104, making it stackable and resistant to shock and vibration as well as ensuring easy installation using standard standoffs inside other enclosures or systems.

Designers also provided the ability to power the board through a host USB connection with the option of using an onboard regulator and external power connector if required. Because power is standard over USB, this was an easy option to offer.

Finally, designers incorporated multiple USB connections including a Type B connector for standard USB cabling. To alleviate concerns associated with loose USB connections in industrial environments, designers provided a USB connector featuring a high-retention design that complies with the Class 1 Division II minimum withdrawal requirement of more than 3 lbs of force. This connector has an orange color-coded insulator to differentiate it from standard USB connectors, eliminating the extra cost of special custom cabling with custom screw locking connections that require solid housing.

For other embedded OEM-type applications, designers included a miniature USB header in parallel with the Type B connector. This small, low-mass, friction-lock, micro-fit connector connects internally from the I/O board to an embedded CPU with its own headers for USB. It can also easily connect to an externally mounted, sealed military connector with its cabling routed to a militarized computer mounted elsewhere.


The USB/104 form factor has proven to be highly versatile, as it can be configured in a single stackable, multistackable, or unstackable system approach. The tiered star architecture and PC/104 mounting style provide many diverse uses and applications not commonly thought of for USB.

For example, the USB/104 board can be mounted at either end of the stack in existing PC/104 enclosures. Although USB/104 exactly conforms to the PC/104 specification in size and mounting holes, its lack of bus pins makes it bus neutral. No matter which bus or CPU is used, almost all computers have onboard USB connections. Because PC/104 boards must have cabling for I/O header connectors anyway, the argument for a dedicated stacking connector is outweighed by the flexibility of interfacing to any lower-cost nonproprietary embedded computer.


An example demonstrating USB/104’s versatility is its ability to enable a high-density I/O system for use in a small, standardized, 1U 19″ rack-mount chassis deployed throughout telecommunications, transportation, audio, entertainment, and other industries. A 1U rack-mount chassis system typically can handle only a few I/O boards.

In this system (Figure 1), up to seven USB/104 boards are hard-mounted flat on standoffs in a 1U rack-mount chassis and wired to a small, commonly available embedded SBC, all without requiring an expensive backplane. Only 5 VDC power supply is needed to power the whole system. This design also has the additional advantage of fanless cooling and natural resistance to shock and vibration.

Figure 1: A 1U rack chassis with seven USB/104 I/O boards demonstrates the new USB form factor’s versatility.

Because new CPU chipsets provide up to eight USB 2.0 root ports, designers created another embedded system (Figure 2) combining PC/104 and USB/104 in two side-by-side, four-board stacks inside a NEMA enclosure. This design aimed to provide a more dense I/O system than a tall PC/104 stack. Even an older embedded PC/104 CPU with four USB ports could have an adjacent USB/104 stack connected to its crowded PC/104 stack to easily increase the number of boards in the system.

Figure 2: By combining PC/104 and USB/104, the NEMA 4 two-stack system provides a more dense I/O solution.

For other form factor systems, the USB/104 I/O boards are flexible enough to be mounted in spare, unused drive bays with a simple wired connection to a motherboard or plug-in backplane-type SBC. This is a cost-effective solution for a system that might need an additional I/O board but is using all of its current plug-in slots.


The USB/104 form factor has several remarkable advantages over traditional I/O products. One of the most noteworthy benefits is its ability to have I/O mounted near what the module is monitoring or controlling. This allowance results in a high degree of integration flexibility and cost effectiveness. Instead of having the CPU and I/O close together, a USB cable can easily run 10 to 15 feet within the enclosure for various applications such as kiosks and computerized OEM equipment.

Because a USB/104 module doesn’t need to be close to the CPU, the module is less susceptible to interference. The protective circuitry and close proximity to the signal source of the I/O board result in virtually noise-free data acquisition. In cases of high-point-count control/acquisition applications, the wiring is greatly simplified by the shorter bulk wiring distance. Only a single USB cable is then routed the longer distance back to the host computer.

Human Machine Interface (HMI) flat-panel systems with simple, small embedded SBCs no longer require space to hold I/O boards. OEM system designers can tuck I/O boards into smaller available spaces within their system enclosures, rather than find a home for a large card cage that must route the mass of I/O cabling to one central location.

The stand-alone nature of USB/104 allows easy system servicing, fault isolation, and board replacement. The typical user can discover how much easier it is to plug in a USB device instead of aligning and installing a PCI slot card. USB/104’s installation and serviceability is much more convenient than the potentially burdensome connection (or disconnection) of the high-density pins and cabling in a conventional PC/104 stack.


Complaints regarding speed and interrupt limitations of USB are about to disappear. The USB 3.0 standard increases the speed 10 times over USB 2.0, includes full-duplex transmission, has new interrupt-driven protocol, and provides additional power available for the I/O device. With the recent release of the USB 3.0 standard and its future inclusion on all CPU chipset vendors’ roadmaps, it is clear that USB will be the low-cost interconnect of choice in the consumer and embedded industries for many years to come.

The USB/104 concept is well aligned for the future of USB embedded I/O systems. ACCES I/O Products offers dozens of USB/104 modules covering a broad spectrum of analog, digital, relay, and serial data acquisition. Embedded systems designers can look to this unique form factor and benefit from USB-based data acquisition for both current and future computer systems.

Number 4 in an ongoing series of articles by employees here at ACCES