With the advent of the serial interconnect such as PCI Express and USB, the most obvious reaction and approach for designing new I/O has been to continue with just offering a connector for the particular form factor or just using USB as an external device in its own housing. Since this is not just a continuation of the progressive evolution of a faster shared bus, this process has been painful for most form factor consortiums. For example, the design decision of how many slots and what type of express lanes (x1, x4, x8, etc.) for the plethora of backplanes, makes the former legacy PCI/ISA slot number decisions seem trivial.

The problem is the desktop world is moving in a different direction than the embedded world. Desktops are moving away from numerous slots in their computer enclosures. These few slots are mostly being reserved for more high-performance applications. Different CPU chipsets are being developed for each niche of server, desktop and laptop. Most of the mundane I/O requirements of desktop and laptop computers are being brought outside the computer through USB ports so that the service issues of adding I/O cards decreases the costs to the manufacturer. Meanwhile, the embedded world still requires numerous I/O boards. Embedded applications are getting smaller as OEM equipment manufacturers are designing more mobile and space-restrictive systems for their markets. Trying to get a computer with a card cage with the right number of slots is becoming a difficult process when trying to use off- the-shelf I/O products.

USB appears to be the obvious solution because of its popularity and ease of use. Unlike a bus, USB can be used in a star configuration where each I/O board does not have to be together or share the same bandwidth. Also, all USB products can interface with all CPU form factors, single board computers and even micro-controllers that support the USB standard. USB is also the best I/O product interface that is easiest to use with both the PC and the MAC.

Most early USB embedded I/O chose to follow the desktop world and create external boxed USB I/O. However, since USB can now be considered a high-performance bus (Acces has models currently available that achieve sustained streaming speeds up to 16 Mbytes/s), the decision was made to design versatile OEM USB form factors that could be used inside and outside enclosures. Since reducing space is a primary goal in embedded applications, the widely used PC/104 form factor was chosen first 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 next new smaller form factor to appear is Pico-I/O, being fostered by the Small Form Factor Special Interest Group or SFF-SIG. This consortium is creating a stacking form factor using the SUMIT interface. This form factor provides four USB ports on the SUMIT A connector. The new tiny form factor is 72 mm by 60 mm, designed to be one half the area of the current PC/104 standard. This size board was also chosen by Acces to create another line of small embedded USB I/O modules (Figure 1).

Features

The first USB OEM board line features the same PCB size and predrilled 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 Pico-I/O-sized USB OEM board follows the same concept for mounting compatibility.

The next feature consideration was the ability to power the board through the PC USB connection or optionally use an onboard regulator and external power connector if required. The Pico-I/O-sized module would only be powered from the USB connections due to its smaller size.

Finally, multiple USB connections were provided. For standard USB cabling, a Type B connector is provided. The concern associated with a loose USB connection in an industrial environment is alleviated by using a USB connector that features a high retention design that complies with the class 1, Div II minimum withdrawal requirement of over 3 pounds of force. 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 micro USB header is provided in parallel with the type B connector. This second, small, low-mass friction lock “micro-fit” connector is useful 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.

The Pico-I/O-sized board follows the same connectivity as the PC/104-sized USB module except the new Micro B connector is used on this tiny I/O module. One of the requirements for the Micro B connector, created to become the cell phone interface standard, was to make a more rugged USB connector for these smaller devices. The metal spring-latched Micro B connector is designed for 10,000 insertions. The availability of cabling for this connector is expected to increase with its future heavy use on PDA and cell phone standardization (Figure 2).