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 (). 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 ","). 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.