[Leapfrog: Industry First]
USB Stacks Up With A Modular Industrial Form Factor
USB moves into the PC/104 form factor with a stacking architecture that parallels EPIC Express.
DEALING THE CARDS The first SBC to implement the StackableUSB architecture is Micro/Sys' SBC1685. The board can handle Pentium and Celeron processors with up to 256 Mbytes of SDRAM. It supports PCI-104 (not PC/104) expansion that's compatible with StackableUSB. Also, it features a Gigabit Ethernet port, four additional USB ports (plus the four StackableUSB ports), two serial ports, PS/2 keyboard/mouse, a parallel port, an IDE controller, and a CompactFlash interface. Onboard video handles resolutions up to 1600 by 1200.
Micro/Sys is starting out with the USB148 digital I/O card (Fig. 2). With 53 digital I/O lines along with timers, this board is comparable to other PC/104 boards in the Micro/Sys product line. The big difference is that it incorporates a 48 MIPS Silicon Labs 8051 microcontroller (with built-in USB support), which serves up its own set of features.
First, microcontrollers of this type often have other interfaces on them. In this case, Micro/Sys brings out the 10bit analog-to-digital converter (ADC), dual UARTs, and serial peripheral interfaces (SPIs). It also ties the microcontroller's I2C to the stackable interface.
Second, the Fast USB interface can handle 12 Mbits/s. This is compatible with the 480-Mbit/s High Speed USB interface on the SBC. However, these two can coexist without any problems anyway.
Finally, the USB interface is brought out, enabling this single board to be used in one of three different environments: the stack; plugged into an external USB port; or as a standalone device using only the onboard microcontroller, which is possible since Micro/Sys provides access to the 8051.
This approach will significantly change the way systems can be delivered and designed. That's because the same hardware can be placed in different locations without worrying about the software interface characteristics. Given wireless USB support, a device may not even be connected directly to the SBC stack.
It will be interesting to see how the standard progresses this year and how it will coexist with EPIC Express. Initially, the StackableUSB boards will be full size, but it's possible to implement smaller cards that would be placed at the top of the stack (Fig. 3). The size and layout of the smaller cards have yet to be fixed.
In theory, two of these cards could fit into the same space as a full-size card. It would be up to vendors to create SBCs or hub adapter cards that would handle these smaller cards. A complete stack, including an SBC, could potentially be built using the smaller form factor. It would not be unreasonable to contemplate a 1/4-size card given the level of integration these days.
The StackableUSB architecture complements EPIC Express, the stackable PCI Express standard. Connectors for each architecture are on opposite ends of the card, so it's possible for cards to support both standards. It's certainly possible to build SBCs that support both.
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