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StackableUSB™ FAQ’s

1. What is StackableUSB™?
StackableUSB™ is a connectivity architecture that allows up to 4 peripheral I/O cards to be stacked on the top and/or bottom of an SBC and communicate with the SBC via USB. Depending on the SBC design it may support a total of 8 peripheral I/O cards, 4 on top and 4 on the bottom.

USB is a point to point or star architecture. StackableUSB™ maintains the Point to point connections required by USB while at the same time allowing the connections to be routed up the stack to the next peripheral in line. The result is a rugged, industrial protocol.

2. Why did we design StackableUSB™?
We felt the need for a modern, easy to use, cost effective stackable interface in the industrial market. Traditionally PC/104 has served this market well. However, as the ISA bus, which is at the heart of PC/104 interfaces, is eliminated from modern chipsets, this aging interfaces is not meeting the market’s needs.

PC/104 has enjoyed enormous popularity for several other reasons. It is physically small and rugged. It is straight forward to program. It does not require complex circuitry to implement I/O boards and is, therefore, inexpensive to use.

StackableUSB™ offers these same advantages while replacing the old ISA with the new USB, which is widely used in the PC industry. USB is included in today’s chipsets in ever increasing numbers which ensures its position as an I/O channel for years to come.

Additionally, StackableUSB™ implements USB 2.0, which in the “high speed” mode is capable of 468mbits per second data transfer. This is considerably faster than the old ISA based PC/104. This increased bandwidth and higher through put is important to the embedded market, especially as faster and faster speeds are coming from the newer generation A/D and D/A.

3. Why is StackableUSB™ needed?
In addition to ISA's obsolescence and the increased throughput of USB 2.0, StackableUSB™ is needed to define and manage a stacking board-to-board communication specification for USB.

Without a defined electrical and mechanical specification for CPU and I/O boards, board manufacturers cannot produce compatible products. Adoption of StackableUSB™ by a standards group enables manufacturers to produce products that “plug and play” in the marketplace.

The universal nature of the StackableUSB™ electrical specification makes it ideal for multiple form factors such as PC/104, EPIC, EBX, miniITX, COM Express, etc. Even newer smaller form factors can be developed, allowing manufacturers to select the form factor best suited for their product.

Placement of the StackableUSB™ footprint on defined form factors is documented in the Recommended Practices section of the Specification.

4. What makes StackableUSB™ unique?
To date, USB has been implemented with a plug/cable assembly interconnect scheme. This limits the environments in which it has been well suited. The stackability in StackableUSB™ allows boards to be securely bolted together, preventing disconnect or separation in harsh environment with high vibration.

StackableUSB™ also brings USB technology to compact spaces. Previously USB was only an option when there was space for USB connectors and cables. Now, two boards can plug together for maximum compactness and density in small enclosures or tight spaces.

5. What markets and applications will use StackableUSB™?
USB I/O has typically targeted the desktop/instrumentation PC market. While providing many I/O devices, the devices have not always been attractive to OEM applications, including many single board computer applications, where USB cabling is inappropriate or a negative feature for the end product.

StackableUSB™ opens up opportunities in industrial control systems, plus mobile, handheld, military, medical and remote communications applications.

6. How many modules can be placed on a stack?
StackableUSB™ uses a robust differential connector. This connector is capable of routing four differential USB pairs up the stack. This allows a system to support at least four USB peripheral cards controlled by one SBC. Optionally an SBC can be designed to support 8 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 8 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 are located near the edge and corner of the board in 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 a 11mm board to board spacing ideal for microcontrollers and ARM processors. The other, shipping in a few months, implements a 16mm 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 characteristic required for USB. Data sheets report the results of the testing.


StackableUSB™ is a trademark of Micro/sys, Inc.



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