William Wong
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ED Online ID #14820 |
February 2, 2007
StackableUSB is a new standard from Micro/sys, designed for industrial environments (USB Stacks Up With A Modular Industrial Form Factor, ED Online #14703, from our February 1 issue). It shares many features with PC/104, PCI-104 and PC/104 Plus, including a 3.6- by 3.8-in. form factor and a stacking architecture. The difference between these standards and StackableUSB is the use of USB 2.0, rather than an ISA or PCI bus, to connect a host to peripherals.
The point-to-point nature of StackableUSB is similar to EPIC Express (More On EPIC Express, ED Online #10983), which also uses a shifting stacking mechanism (Fig. 1). It also has a similar limitation, since both StackableUSB and EPIC Express have a base unit that starts with four single point-to-point links. (Note that EPIC Express also has an x4 version that starts with two x4 PCI Express links and four x1 PCI Express links.)
A StackableUSB system has some advantages over EPIC Express when it comes to the number of boards, because this standard allows a USB hub to fit in the middle of a stack. This means a 5-board system (SBC plus 4 IO boards) can be extended to a 9-board system. That is more than a typical PC/104 system, which averages 2.5 boards. In addition, the StackableUSB architecture can support half-width cards (Fig. 2). There is even the potential to have a half-width single-board computer for a very compact, modular system.
StackableUSB has a number of other advantages, including a smaller connector size while still retaining the capability to run at USB 2.0's 480-Mbit/s rate. The architecture also takes advantage of the plethora of microcontrollers with USB device support. These microcontrollers are available in a range of configurations, from 8-bit to 32-bit versions. These typically have more interfaces than just a USB port, and the additional peripherals can be used on the StackableUSB interface boards (Fig. 3).
StackableUSB offers some other interesting options that should show up in future products. For example, the ExpressCard standard (ExpressCard Replaces PCMCIA, ED Online #10183) is based on USB and PCI Express. An ExpressCard will normally have only one of these interfaces, although in theory it could have both. With a system that combines EPIC Express' PCI Express support and StackableUSB's USB support, a more viable approach is simply to support only USB-based ExpressCards with StackableUSB.
Likewise, StackableUSB can be used to support a range of USB peripherals, from memory sticks to wireless interfaces. These are readily available for laptops and desktops, and they can be utilized in embedded StackableUSB systems with minimal overhead — often just one connector, versus the numerous and costly (relative to the cost of just the connector) interface chips often required for ISA or PCI interfaces.
Stay tuned for more on StackableUSB as real products start to ship. Building a system based on these products should be as easy as using PC/104 boards. The results are likely to be cheaper, more reliable and less costly in the long run.
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