[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.
The stackable PC/104 form factor has proved itself indispensible in numerous applications. But its Industry Standard Architecture (ISA) is moving toward oblivion as the number of chips that can directly interface with it nears zero.
USB, one of the interfaces replacing ISA, is ubiquitous on PCs and laptops. Embedded systems tend to be the last to jump on the bandwagon, though USB will be found on most PC/104 single-board computers (SBCs). What's lacking is a stackable version of USB with the ruggedization of a PC/104 stack.
Micro/Sys' new small industrial form factor, StackableUSB, looks to fill the void. The company is a big supporter of PC/104, so it's no surprise that the new architecture fits the 3.550- by 3.775-in. PC/104 form factor. It even uses the same mounting holes. And, the stacking architecture looks very much like that of EPIC Express (see "More On EPIC Express").
In this case, the two USB differential pairs (USB input and output) are routed to the on-board USB device while the remaining three sets are shifted left one position and routed to the next board in the stack (Fig. 1). The last position remains unconnected, so eventually, you run out of direct connections between the host and the top board.
The design doesn't require any on-board USB hubs, only a USB device. In general, each board will have a single USB device, typically a USB-device-capable microcontroller. It's possible to connect multiple devices on a StackableUSB card using an on-board USB hub.
By taking the StackableUSB approach, up to four USB devices can be connected directly to the host. Adding a USB hub board into the stack allows for an additional four boards. Trying to add yet another USB hub board pushes the limits of the USB system and the power that can be provided up the bus, though.
The use of differential USB signals instead of single-ended signals improves reliability. Likewise, the StackableUSB socket and the fact that the boards are bolted together make the architecture significantly more reliable than conventional USB cable connections. It also suits StackableUSB for environments where conventional USB connections would be impractical.
The implementation uses a 28-pin, surface-mount plug-and-socket (see "StackableUSB Pins,"). An independent set of center pins provides ground. The ground connection can handle 9.5 A, and each pin can handle up to 2 A. Four 5-V pins deliver up to 8 A. The usual limit for a USB device is 500 ma.
Boards using the StackableUSB sockets can't support PC/104 because the sockets are in the same location as the PC/104 ISA bus. In theory, it can work with PCI-104 (PCI connector only) and EPIC Express, whose connector is on the opposite side.
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