The PC/104 Consortium, www.pc104.org, is a technical organization that creates, maintains, and distributes specifications supporting the PC/104 architecture, which includes the bus configurations PC/104, PC/104-Plus, PCI-104, PCI/104-Express, and PCIe/104 (see Figure 1). The Consortium also supports the form factors 104, EPIC, and EBX (see Figure 2). These specifications embody the stackable PC from ISA to PCI to PCI Express.

The stackable PC, as its name implies, provides a modular, stackable architecture that leverages the wealth of components, software, development tools, and overall acceptance of the desktop PC market for embedded system designers. It has exactly followed the desktop PC starting with the ISA bus in PC/104, ISA and PCI bus in PC/104-Plus, PCI bus alone in PCI-104, PCI and PCI Express in PCI/104-Express, and PCI Express bus alone in PCIe/104.

ISA bus, long gone from desktop PCs, is still very much in demand in embedded systems. Many Consortium members and component suppliers continue to manufacture devices and boards for this popular embedded standard. PCI bus systems built around PCI-104 are fielded for demanding embedded applications around the world. PCI Express with its software compatibility to PCI bus is being welcomed by the embedded market. The Consortium’s PCIe/104 Type 1 features PCI Express x1 and x16 links with USB 2.0 while Type 2 has PCI Express x1 and x4 links with USB 3.0 and SATA to support the latest peripheral devices.

The PC/104 Consortium introduced its PCI Express version of the stackable PC in 2008 with a connector that was tested to the PCI Express 2.5 Giga-Transfers/Second (GT/s) rate for x1 and x16 links. In 2011 Type 2 enhanced PCIe/104 by offering PCI Express x4 links, SATA, and USB 3.0 in place of the x16 link. Since all peripherals support both up and down stacking, we are seeing manufacturers build CPUs with only one connector and even some hosts with Type 1 stacking down and Type 2 stacking up. This is possible because the connectors used for PCIe/104 are surface-mounted and are not required to have the same connector configuration on the top and bottom of the CPU host module.

PCI Express has advanced from Gen 1 to Gen 2 at 5.0 GT/s and Gen 3 at 8.0 GT/s. This is pretty fast for a stacking architecture and requires a high-quality connector. PC/104 architecture was tested and, as this article is being written, the PC/104 Consortium members are voting on a new revision of the PCIe/104 specification that incorporates PCI Express Gen 2 and 3 in the stackable PC. This revised specification provides the guidelines to ensure embedded system designers that building with PCIe/104 will be as easy, rugged, and reliable as building with PCI-104 and PC/104.

To find out more about PC/104 or download the specifications for free, visit the PC/104 Consortium at www.pc104.org, where you’ll see the wealth of PC/104 architecture products. The Consortium has recently added videos with even more useful information on PC/104 architecture.

Jim Blazer, Vice Chairman and CTO of RTD Embedded Technologies, has been involved with PC/104 technology and the PC/104 Consortium for 20 years. He has served on the Consortium’s board of directors and technical committee and has chaired both. He is currently a member of the technical committee and is actively involved in PCIe/104 specification development.