USB is the de facto peripheral interconnect outside the box. Inside, though, it’s been a different story. This year, USB has been internalized with a range of products and platforms becoming more popular. My two choices for “best” reflects this change, with Stackable- USB and the MiniBlade standards. StackableUSB (www.stackableusb.org) has its own organization behind it, and the MiniBlade standard is promoted by the Small Form Factor SIG (sff-sig.org).
At 480 Mbits/s, USB fills the growing interconnect space between slower interfaces such as I2C, SPI, and CAN and higher-speed interfaces such as Ethernet and PCI Express. The plethora of USBcapable microcontrollers on the host and client side makes USB the ideal mechanism to interconnect devices inside the box as well as outside where PC peripherals have dominated this space.
One of the main changes this year is USB’s ruggedization. The USB connectors used for external connections aren’t suitable for internal use, but these new offerings are.
Micro/sys (www.embeddedsys.com), the driving force behind StackableUSB (see “Micro/sys Dishes Out Stackable USB For Embedded IO” at www.electronicdesign.com, ED Online 14820), has the first group of motherboards and peripherals available (Fig. 1). Micro/sys is also a PC/104 board vendor, which is the space StackableUSB targets.
StackableUSB uses a Samtec connector with a center blade that provides a solid ground connection. It also features eight USB links and an I2C interface. The I2C multidrop interface is designed to connect to all boards within a stack. The pointto- point USB links are meant to be used one at a time by each board in the stack. Each board uses the first USB link on the incoming connector and routes the unused links so the first unused incoming link is the first link in the outgoing connector.
StackableUSB boards can come in various sizes, allowing for compact platforms when board real estate isn’t needed for additional hardware. As with other USB-based solutions, expansion can move outside the board. It’s even possible to build a USB peripheral based on StackableUSB boards, foregoing an embedded StackableUSB motherboard.
The MiniBlade standard is new from the Small Form Factor SIG, but it has a large group of vendors supporting it. Its initial champions are Samtec (www.samtec.com) and SiliconSystems (www.siliconsystems.com), whose SiliconBlade flash-memory USB storage device (Fig. 2) will likely lead the way in product shipments related to the standard.
The latching, 40-pin Samtec socket can host a range of high-speed serial interfaces, including USB, SATA, and PCI Express. MiniBlade modules only need to support one of these interfaces. The same is true on the host side, so not all modules will work with all sockets. Modules and motherboards can support multiple interfaces.
The connectors handle hot insertion, but that’s not so much the case for hot removal. Removing a module takes two hands or a special tool. A single socket will probably be the norm. However, multiple socket motherboards can be quite useful. The motherboard would typically hold the host processor and any additional switch chips needed to handle multiple sockets. The sockets could also provide a useful, rigid platform for devices such as cameras.
StackableUSB and MiniBlades deliver a rugged version of USB inside the box. Several cabled solutions from companies such as Acces I/O Products provide similar functionality, with PC/104-size modules with a high-retention USB connector on the side. Samtec offers a collection of high-retention sockets in horizontal and vertical form factors. The advantage of USB cables is the ability to move the modules away from the host and closer to the devices being controlled or monitored.
Will anyone mix cabled USB, StackableUSB, and MiniBlades? Possibly. There’s nothing to prevent it. Meanwhile, external peripherals will continue the growth of USB and its variants, such as Wireless USB, but now USB will be even more of a factor inside the box.