StackableUSB is a new standard from Micro/sys, designed for industrial environments (USB Stacks Up With A Modular Industrial Form Factor, ED Online #14703, from our February 1 issue). It shares many features with PC/104, PCI-104 and PC/104 Plus, including a 3.6- by 3.8-in. form factor and a stacking architecture. The difference between these standards and StackableUSB is the use of USB 2.0, rather than an ISA or PCI bus, to connect a host to peripherals.
The point-to-point nature of StackableUSB is similar to EPIC Express (More On EPIC Express, ED Online #10983), which also uses a shifting stacking mechanism (Fig. 1). It also has a similar limitation, since both StackableUSB and EPIC Express have a base unit that starts with four single point-to-point links. (Note that EPIC Express also has an x4 version that starts with two x4 PCI Express links and four x1 PCI Express links.)
A StackableUSB system has some advantages over EPIC Express when it comes to the number of boards, because this standard allows a USB hub to fit in the middle of a stack. This means a 5-board system (SBC plus 4 IO boards) can be extended to a 9-board system. That is more than a typical PC/104 system, which averages 2.5 boards. In addition, the StackableUSB architecture can support half-width cards (Fig. 2). There is even the potential to have a half-width single-board computer for a very compact, modular system.
StackableUSB has a number of other advantages, including a smaller connector size while still retaining the capability to run at USB 2.0's 480-Mbit/s rate. The architecture also takes advantage of the plethora of microcontrollers with USB device support. These microcontrollers are available in a range of configurations, from 8-bit to 32-bit versions. These typically have more interfaces than just a USB port, and the additional peripherals can be used on the StackableUSB interface boards (Fig. 3).
StackableUSB offers some other interesting options that should show up in future products. For example, the ExpressCard standard (ExpressCard Replaces PCMCIA, ED Online #10183) is based on USB and PCI Express. An ExpressCard will normally have only one of these interfaces, although in theory it could have both. With a system that combines EPIC Express' PCI Express support and StackableUSB's USB support, a more viable approach is simply to support only USB-based ExpressCards with StackableUSB.
Likewise, StackableUSB can be used to support a range of USB peripherals, from memory sticks to wireless interfaces. These are readily available for laptops and desktops, and they can be utilized in embedded StackableUSB systems with minimal overhead — often just one connector, versus the numerous and costly (relative to the cost of just the connector) interface chips often required for ISA or PCI interfaces.
Stay tuned for more on StackableUSB as real products start to ship. Building a system based on these products should be as easy as using PC/104 boards. The results are likely to be cheaper, more reliable and less costly in the long run.
Q&A With Micro/sys
Micro/sys is the original developer of the StackableUSB standard, so I decided to pick their brains to find out more on this new architecture.
| Electronic Design: | Is the StackableUSB a standard? What type of licensing terms are available? |
| Micro/sys: | The specification defines the standard, and therefore StackableUSB is a standard. It is available for licensing. Details are provided at stackableusb.org. |
| ED: | USB has been found on SBCs for many years. How did the demand for more USB peripherals in embedded systems drive the design of StackableUSB? |
| Micro/sys: | The demand for USB itself was not the motivator for StackableUSB. The demand was actually for a replacement for ISA that is of similar performance and cost. ISA is becoming more difficult and costly to include in SBC designs as it approaches obsolescence. Other faster, more expensive, and complex busses have failed to displace ISA. We feel that USB is the best choice to replace ISA. |
| ED: | StackableUSB uses a new connector. Why was it designed, and what features does it bring to the embedded USB environment? |
| Micro/sys: | The Q2 connector is rugged, limits crosstalk between differential pairs, and supports high frequency signaling. |
| ED: | How important is StackableUSB's bolt-down orientation in an embedded environment? |
| Micro/sys: | This is addressed in the StackableUSB FAQs. |
| ED: | StackableUSB uses differential signals. What advantages does this approach have, and what impact does it have on system design? |
| Micro/sys: | USB uses a single differential pair per point-to-point connection between the USB host and each USB device. Therefore USB dictates the use of differential signaling. Differential signaling is intended to reduce the impact of common mode noise on digital signals. |
| ED: | Will there or could there be a version of StackableUSB that uses single-ended signals? What might the advantages or disadvantages be? |
| Micro/sys: | No. USB does not support single-ended signaling. |
| ED: | The idea for a stackable USB architecture has been around for a while, but PCI Express beat it out the door with the stackable EPIC Express standard. Why did StackableUSB take longer to come to fruition? |
| Micro/sys: | Good discernment is an ongoing process, and market opportunities change as time passes. USB had been out for a while when the more complex, high-bandwidth PCI Express emerged. As EPIC Express implemented PCI Express for embedded systems, the concern with USB was that it would be phased out of CPUs and chip sets, just as ISA was quickly being eliminated. But USB kept getting stronger with USB 2.0, USB On-the-Go, and Wireless USB. More significantly, USB moved beyond the PC world to other CPU cores such as microcontrollers and ARM XScale processors. This expanding presence of USB could not be ignored. |
| ED: | Seven adapter boards seems like more than enough expansion, given that most PC/104-style stacks have an average of 2.5 boards. Still, would it be possible for SBC designers to provide another stack on the bottom of an SBC for further expansion? |
| Micro/sys: | The StackableUSBTM specification does allow for support for up to 10 peripherals from a single SBC without the use of hub cards on top and bottom. |
| ED: | How well does StackableUSB mix with EPIC Express? Would this combination allow full support of the ExpressCard standard that mixes USB and PCI Express x1? |
| Micro/sys: | StackableUSB in its 15.24-mm board-to-board spacing form is fully compatible with Epic Express. In other words, it is possible to design an SBC that is capable of controlling both StackableUSB peripherals and Epic Express peripherals simultaneously. It is further possible to design a single peripheral that supports both interfaces. |
| ED: | The possibility of smaller StackableUSB card form factors has been mentioned. What is the status of these, and what kinds of options might be available? |
| Micro/sys: | The most recent revision to the StackableUSB specification specifies the ½-size peripheral card and also allows up to two ½-size stacks on both the top and bottom of the SBC. We envision the emergence of smaller formats. |
| ED: | Could the StackableUSB architecture be implemented using even smaller connectors for very small embedded systems? |
| Micro/sys: | In theory, yes, and the beauty of StackableUSB is the growth that is possible in the future. That being said, planning and care need to be taken to protect the integrity of the StackableUSB standard for the user. |
For more information, go to Micro/sys, PC/104 Consortium, PCMCIA, and StackableUSB.