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The PC/104 evolution: PCIe in the mix

Posted: 15 Oct 2008 ?? ?Print Version ?Bookmark and Share

Keywords:evolution PC/104? PCIe? PCI Express?

The PC/104 bus is an ISA-compatible, 3.550-inch x 3.775-inch, stackable board-level architecture that evolved from the MiniModule mezzanine bus fielded by Ampro Computer in 1987 and was standardized under the auspices of the PC/104 Consortium in 1992. It grew over time into a family of architectures, with the advent of PC/104-Plus (ISA bus and PCI bus) in 1997 and PCI-104 (PCI only) in 2003. The latest incarnation of the form factor adds PCI Express (PCIe) to the mix, in three different specifications, all announced in spring 2008: PCI/104-Express and PCIe/104 from the 60-member consortium, now known as the PC/104 Embedded Consortium; and Express104 from a new trade group called the Small Form Factor Special Interest Group (SFF SIG).

PCI/104-Express and PCIe/104 take the same approach as the previous generation of PC/104-compatible form factor specifications did in their transition from ISA to PCI. PCI/104-Express combines PCIe with a PCI bus, and PCIe/104 provides only PCIe. The common high-speed, high-density, 156-pin PCIe connector used for both accommodates four x1 (single-lane) PCIe links and one x16 (16-lane) link. The wide link can optionally be configured as two x8 links, two x4 links or two SDVO (Serial Digital Video Out) interfaces. Other signal lines are defined for an SMBus (System Management Bus) and control signals for an ATX-class power supply.

The SFF takes a different approach entirely. The group, formed in September 2007 by Octagon Systems, Samtec, Tri-M Systems and Engineering, VIA Technologies and WinSystems, and now 18 members strong, also incorporates PCIe into a new connector on PC/104 form factor boards, but couples this with multiple low- and medium-speed I/O and utility interfaces rather than PCI.

An Express104 board can have one or two 52-pin Sumit (Stackable Unified Module Interconnect Technology) connectors, based on a complementary specification for a form-factor-independent connector that was announced at the same time as Express104. An A connector provides two x1 PCIe links and one x4 PCIe link, plus three USB 2.0 interfaces, an LPC (Low Pin Count) Bus, dual SPI (Serial Peripheral Interface)/uWire channels, SMBus and a set of ExpressCard interface signals. A B connector provides another set of x1 and x4 PCIe links. Express104 also supports an optional configuration that accommodates legacy PC/104 boards.

Signature approach
Both the PC/104 Embedded Consortium and SFF SIG have their own approaches to various issues for making a smooth transition to PCIe. Both, for example, have ways of handling backward compatibility, their own sets of board stacking rules and guidelines, and automatic routing and alignment techniques for PCIe lanes and the like. For the former group, the focus of legacy compatibility is on PC/104-Plus and PCI-104 and the PCI bus; for the latter group, the compatibility focus is on PC/104 and the ISA bus.

As far as stacking philosophy, PCI/104-Express allows stacking boards above and below a host CPU board, but the PCIe and PCI interfaces must reside on different sides. Express104 boards, in contrast, stack in only one direction "to reduce cost and complexity of the support circuitry, plus allow support of faster signaling speeds," according to the SFF SIG. Nevertheless, Express104 stacks that include legacy PC/104 boards are allowed to stack them in the opposite direction from that of the PCIe boards.

With its multiplicity of interfaces, Express104 stacking is a bit more complicated. As with PCI/104-Express, however, boards with wider PCIe links must reside closer to the host CPU than those with narrower links. Next in order are boards with the specification's other native interfaces, followed at the top by legacy PC/104 boards.

Great expectations
Express104 has yet to see its first product introductions, but some announcements are expected at the Embedded Systems Conference later this month, with availability during Q4 08. A number of PCI/104-Express board-level products, in contrast, are already available.

Connect Tech, for example, is fielding an 8-port serial communications board supporting RS-232, RS-422 and RS-485 links, as well as a carrier/adapter board that lets a PCI/104-Express board be inserted into a conventional PCIe system slot. RTD Embedded Technologies, in turn, has a dual-channel Gigabit Ethernet (GbE) board, a high-speed, 48-line digital I/O board, and an 88W power supply board on the market for PCI/104-Express.

Elsewhere, Digital-Logic AG has nearly a dozen products supporting PCI/104-Express available. These include a GbE board with dual USB interfaces and a quad GbE board with four RJ45 interfaces; single-board computers based on Intel Core 2 Duo L7400 and Atom Z510/530 processors; a dual-screen video processor; MiniCard carrier; ExpressCard carrier; video frame grabber; GSM voice communications board; Firewire board with one IEEE1394A and two IEEE1394B links; and a board that provides floppy and hard disk drive interfaces.

- David Lieberman
EE Times

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