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1. What is StackableUSB? StackableUSB is a means of mechanically and electrically establishing USB communication between a single board computer (SBC) and a peripheral I/O card stacked onto the top and/or bottom of the SBC. Depending on the SBC design, the CPU can support a total of 10 peripheral I/O cards, 5 on top and 5 on the bottom, without the use of a hub board. StackableUSB maintains the point-to-point connections and star topology required by USB and at the same time allows the connections to be routed up the stack to the next peripheral in line. The result is a rugged, industrial system using USB protocol. 2. Why did we design StackableUSB? There is a need for a modern, easy to use stackable system architecture for industrial users. Traditionally, PC/104 has served this market. However, with the ISA bus interface being eliminated from current PC chipsets, this aging protocol needs a successor. USB is currently available in almost every popular chipset and is on the PC chipset roadmap for years to come. Additionally, USB is included in many microcontrollers and ARM processors, making it one of the most universally used available interfaces. StackableUSB incorporates the same advantages as PC/104 – physically small and rugged, straightforward and easy to program, simple solutions for designing user-defined I/O boards, and inexpensive to use. All this, plus StackableUSB 2.0 runs faster. In the "high speed" mode USB 2.0 runs at 480mbits per second data transfer. This increases the bandwidth and speeds found in the traditional PC/104 I/O boards. This new speed allows systems to take advantage of the newer, faster I/O chips being released in areas like A/D and D/A. 3. Why is StackableUSB needed? StackableUSB defines a stacking board-to-board communication specification for USB that board manufactures can adopt. Without this electrical and mechanical specification, CPU and I/O board manufacturers cannot design and manufacture product that is interchangeable. Adoption of the StackableUSB system architecture enables manufacturers to produce products that are interoperable. The universal nature of the StackableUSB electrical and mechanical specification makes it ideal for adoption on multiple form factors such as PC/104, EPIC, EBX, miniITX, COM Express, etc. Even newer, smaller form factors will be ideal, as each form factor will allow board manufacturers to match the appropriate I/O to CPU combination. Placement of the StackableUSB footprint on defined form factors is documented in the Implementation and Recommended Practices section of the Specification. 4. What makes StackableUSB unique? To date, USB has been limited to systems where a plug/cable interconnect scheme was appropriate. This has limited the applications where USB has been well-suited. StackableUSB allows boards to be securely connected, without cables, in a method that will prevent them from separating or disconnecting in rugged environments. StackableUSB brings USB to compact spaces. Previously USB was an option only when there was room for USB connectors with loose cables. Now, up to eleven boards can plug together to form a dense and compact system that will fit in small enclosures or tight spaces. There is also increased system reliability, and increased ease of field service for systems due to elimination of bulky cables. StackableUSB provides several system-enhancing features as well. The “stack”, from the CPU host, provides more power for the USB devices, thus reducing the need for separate power sources for each device. The “stack” has 5.0V and 3.3V available. Additionally, StackableUSB provides an I2C bus and a system reset signal. 5. What markets and applications will use StackableUSB? USB I/O devices have typically targeted the instrumentation market which has been “connected” to a desktop PC. These devices have not always been attractive to OEM users, including many using single board computer users, because of packaging and logistic issues. StackableUSB opens up opportunities in industrial control systems, plus mobile, hand-held, military, medical and remote communications applications. 6. How many modules can be placed on a stack? StackableUSB uses a robust connector. This connector is capable of routing five differential USB pairs up the stack. This allows a system to support at least five USB peripheral cards controlled by one SBC. Optionally an SBC can be designed to support 10 USB peripheral I/O devices. The architecture supports mounting the SBC on the top or on the bottom of the stack, or in the case of the 10 port SBC, both simultaneously. 7. Is there a special alignment or slot scheme for StackableUSB modules? The locations of the top and bottom stacking connectors are specified. They are relatively small and have been placed near the edge and corner of the board in order to minimize the impact on peripheral board layout. They are keyed and the defined location guarantees proper mounting when the boards are bolted together. 8. Are the StackableUSB connectors available now and if so from whom and what size are they? Currently there are two connectors available from Samtec. One, shipping now, implements an 11.26mm board-to-board spacing ideal for microcontrollers and ARM processors. The other, shipping in a few months, implements a 15.24mm board-to-board spacing suited for faster speed CPUs requiring CPU heat sinks or connectors flush mounted on the I/O boards. 9. Are there reliability reports on the performance of the StackableUSB connector? Yes. Samtec has done testing for the connector in simulations that measure the electrical characteristics required for USB. Data sheets report the results of the testing.

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