Originally appeared in RTC Magazine
The embedded computer I/O world has always adapted to a particular CPU-based form factor such as PC/104, PMC, cPCI, VME or PCMCIA. The first form factor of slot cards was rooted in the desktop. All these form factors were based on a similar parallel bus but with the appropriate connectors to serve a particular market?s purpose.
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).
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).
USB/104 and USB/PICO Introduction
Traditionally, USB I/O vendors have simply pulled an existing USB PCB board out of its enclosure and voila, called it OEM. However, these new preconceived USB OEM designs have been named USB/104 and USB/PICO.
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 unstacked 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 “Bus connector neutral.” No matter what bus is used, or CPU chosen, almost all computers have onboard USB connections. Since PC/104 boards must have cabling for I/O header connectors anyway, the argument for a dedicated stacking connector in many applications is outweighed by the flexibility of interfacing to any lower cost non-proprietary embedded computer.
The Pico-I/O form factor is designed to be the younger, smaller brother of PC/104. The USB/PICO OEM module will also have similar advantages to USB/104. Although less I/O can be packed in the given PCB space, it has the added advantage of fitting in a smaller system space and using less power than a PC/104-sized module (Figure 3).
Another example of USB/104 and USB/PICO versatility is the capability to design a high-density I/O solution for low height clearance applications. The boards can be mounted unstacked flat on standoffs in any low height enclosure and then wired to a small, embedded, single board computer (SBC)—all without the need for an expensive backplane and by using a commonly available embedded SBC with USB ports. Only a small 5 VDC 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. Twice the number of different modules could be used if the smaller USB/PICO I/O boards are used. The great thing about this is a system designer can use USB/104 and USB/PICO modules together in the same application with any computer (Figure 4).
Since new CPU chipsets now provide from 8 up to 16 USB 2.0 root ports, an embedded system can be designed combining multiple USB/104 or USB/PICO board stacks side-by-side inside a NEMA enclosure. This design concept allows a more dense I/O solution than a single tall I/O module stack. Even an 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. USB/PICO would also have this increased flexibility but inside an even smaller space. Both USB/104 and USB/PICO finally have the additional advantage of being able to use shorter stand-offs, depending on the board, because the spacing in the stack is not controlled by the stacking connector of either PC/104 or SUMIT. This leads to the ability of squeezing an extra board into a larger stack within a shorter height enclosure.
For other non-stacking form factor systems, both the USB/104 and USB/PICO 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 where all the current plug-in slots are used up.
The USB/104 and USB/PICO form factors have a number of common advantages over existing bus-based I/O products. One of the most noteworthy is the ability to have the I/O module mounted near where the data acquisition or control is taking place. This proximity to sensors results in a high degree of integration flexibility and is significantly cost-effective. Instead of the computer 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 or USB/PICO modules do not need to be close to the CPU, the modules are less susceptible to noise. Combined with protective circuitry, and close proximity to the signal source of the I/O board, the result is virtually interference-free data acquisition.
In cases of high point count control or acquisition applications, the wiring is greatly simplified by the shorter bulk wiring distance. A single USB cable can be more economically protected and then routed the longer distance back to the host computer. Simple flat panel touch-screen computers such as Human Machine Interface (HMI) with small, embedded SBCs will no longer require space to hold I/O boards. OEM system designers also can now pack I/O boards into smaller available spaces within their system housings, rather than painfully trying to find a home for a big card cage that must route the large mass of I/O cabling to one central location.
The stand-alone nature of USB/104 and USB/PICO allows easy system servicing, fault isolation and board replacement. The typical desktop 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. The convenient installation and serviceability of these USB modules compared to the more challenging task of connecting (or disconnecting) bus connectors makes USB I/O an easy and natural fit for numerous embedded applications.
Future of Embedded USB
Originally, the most beneficial feature 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 related to speed limitations and the lack of an interrupt or mechanism to alert the host without constant servicing. Fortunately, the next approved Super Speed 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 for the I/O device by using a higher pin count connector.
With the recent release of the USB 3.0 standard and its future inclusion on all CPU chipset vendor’s roadmaps, it is clear that USB will be the low-cost I/O solution both for the consumer and the embedded industry for many years to come. USB 3.0 will basically serve equivalent performance to 1x PCI Express tasks over wire. USB 3.0, however, will not necessarily eliminate USB 2.0 because USB 2.0 is less expensive to implement and is compatible to USB 3.0. USB 2.0 when plugged into USB 3.0 simply does not access the extra deeper pins in the host USB 3.0 cable connector. The embedded USB space will likely only use USB 3.0 when the speed, interrupt and power requirements of the I/O device need its advanced features, thus justifying its increased cost. Meanwhile, existing USB 2.0 devices with typical data acquisition and control applications will be supported with their smaller inexpensive cabling and interfacing.