Number 6 in an ongoing series of articles by employees here at ACCES

Originally appeared in

USB/104 Changes Embedded OEM I/O Board Paradigm

As PC-based computers move away from the PCI/ISA bus for I/O cards, the USB interface can provide more than just a substitute for traditional plug-in boards.

Embedded and industrial I/O has traditionally followed the desktop in offering slot bus connectors for I/O cards to plug into motherboards or passive backplanes. Other embedded form factors have also flourished such as cPCI, VME, PC/104, PMC or PCMCIA. Although electrically similar, each bus has its own connectors and mechanical footprint to fill its own technology market niche. Each standard requires CPU motherboards designed for its own particular form factor and only works with I/O cards in the same form factor. Many applications are currently limited by the space, number and type of I/O slots available in any given computer system. Today’s OEM equipment designer is facing a common and constant struggle trying to fit computer and I/O requirements into the given space he has available.

USB has enjoyed a rapid, tremendous increase in use in the consumer space over the last few years. Increasingly, industrial and embedded I/O vendors have realized the advantage USB brings to the industrial marketplace and have released a multitude of industrial I/O boards packaged into a variety of enclosure types. USB I/O solutions are quickly and simply realized by merely connecting the I/O by wire to any computer with an available USB port. No longer does a particular motherboard require I/O to be built to its bus form factor.

ACCES I/O Products has decided to take the next step in regards to USB embedded I/O solutions. Since USB can now be considered as a high-performance bus (ACCES has models currently available which achieve sustained streaming speeds up to 16 MB/s), the decision was made to design a versatile OEM USB form factor that could be used inside and outside its own enclosure. Since reducing space is a primary goal in embedded applications, the widely used PC/104 form factor was chosen as a starting reference point in the design of this new USB I/O concept. The small size of PC/104, the rugged design, hundreds of boards and dozens of manufacturers made this an easy choice.


The ACCES USB OEM board features the same printed circuit board (pcb) size and pre-drilled mounting holes of PC/104 which allows easy stacking and resistance to shock and vibration. Additionally, this ensures easy installation using standard standoffs inside other enclosures or systems.

The next feature consideration was the ability to power the board through the PC USB connection or optionally use an on-board regulator and external power connector if required.

Finally, multiple USB connections were provided. For standard USB cabling, a Type B connector is provided. Concern associated with a loose USB connection in an industrial environment is alleviated by using a USB connector which features a high retention design that complies with the class 1, Div II minimum withdrawal requirement of over three-pounds of force. This connector has an orange color-coded insulator to differentiate it from standard USB connectors. This also alleviates the extra cost of special custom cabling with custom screw locking connections which require a solid housing to interface to. For other embedded OEM type applications, a miniature USB header is provided in parallel with the type B connector. This second, small, low-mass friction lock ‘micro-fit’ connector is excellent for connecting internally from the I/O board to an embedded CPU with its own headers for USB. Also, connecting to an externally mounted, sealed military connector with its cabling routed to a militarized computer mounted

elsewhere becomes trivial.

USB/104 form factor is proven versatile

Traditionally, USB I/O vendors have simply pulled an existing USB pcb board out of its enclosure and called them OEM. With the ACCES approach, the resulting pre-conceived USB OEM design has been named USB/104. The USB/104 form factor has proven to be one of the most versatile I/O form factors currently available. It can be configured in a single stackable, multi-stackable or un-stackable system approach. The tiered-star architecture and PC/104 mounting style has provided many diverse uses and applications not commonly thought of for USB.

For example, in an existing PC/104 system, the USB/104 board can be mounted at either end of the stack in existing PC/104 enclosures. Although mechanically exact to the PC/104 specification in size and mounting holes, the lack of bus pins makes the USB/104 ‘PC bus neutral’. No matter what bus is used, or CPU chosen, almost all computers have on-board USB connections. Since 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 non-proprietary embedded computer.

Another example of USB/104 versatility is the capability to design a high density I/O solution for use in a small, standardized, 1U 19-inch rack mount chassis widely used throughout the telecommunication, transportation, audio, entertainment and other industries. Typically a 1U rack mount chassis system can handle only a couple I/O boards. Up to seven USB/104 boards have been hard mounted flat on standoffs in a 1U rack mount chassis and then wired to a small, embedded, single board computer (SBC) - all this without the need of an expensive backplane and by using a commonly available embedded SBC with USB ports. Only a small 5Vdc power supply is required to power the whole system. This design also has the additional advantage of fanless cooling and high natural resistance to shock and vibration.

Since new CPU chipsets now provide up to eight USB 2.0 root ports, an embedded system was designed combining PC/104 and USB/104 in two side-by-side, four-board stacks inside a NEMA enclosure. This design was aimed to provide a more dense I/O solution 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 in order to easily increase the number of boards in the system.

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 very cost effective solution for a system that possibly needs an additional I/O board, but all the current plug-in slots are used up.


The USB/104 form factor has a number of remarkable advantages over traditional I/O products. One of the most noteworthy is the 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 is exceptionally cost effective. Instead of the CPU and I/O close together, a USB cable can easily be run 10-15 feet within the enclosure for such varied applications as kiosks and computerized OEM equipment. Because the USB/104 module doesn't need to be close to the CPU, the module is less susceptible to interference. Combined with protective circuitry, and close proximity to the signal source of the I/O board, the result is 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. HMI (Human Machine Interface) flat panel systems with simpl

e, small, embedded SBC’s no longer will require space to hold I/O boards. OEM system designers can now tuck I/O boards into smaller available spaces within their system enclosures, rather than finding 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 PC consumer has already found out how much easier it is to plug in a USB device rather than aligning and installing a PCI slot card. How much more convenient is the installation and serviceability of USB/104 compared to the potentially burdensome connection (or disconnection) of the high density pins and cabling in a PC/104 stack?


The biggest virtue of USB was its ability to have I/O boards powered by USB ports. This was also a big negative when I/O boards exceeded the power available from the host USB ports. Other big complaints were regarding speed limitations and the lack of an interrupt or mechanism to alert the host without constant servicing. Fortunately, the next approved Super Speed USB 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 vendor’s roadmaps, it is clear and guaranteed that USB will be the low cost I/O solution both for the consumer and the embedded industry for many years to come. The USB/104 concept of ‘OEM System SPACE Flexibility’ is aligned for the future of USB embedded I/O solutions.

Acquisition, Control, Communication: Engineering and Systems
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