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
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
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
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
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
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
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.