A simple, elegant solution enables outstanding legacy support

One of the biggest challenges facing the embedded community during the past few years has been dealing with older technologies as new technologies come online. Some embedded consortia have taken the approach of making a clean break from the past. They run headlong into adopting new technologies while removing any vestige of older technologies from their standards. They proclaim “LEGACY FREE” to indicate how proud they are that their standards have eliminated serial ports, the ISA and PCI buses, and other “obsolete” technologies.

SFF-SIG, among others, has decided to embrace the past instead of rejecting it. Our Stackable Unified Modular Interconnect Technology (SUMIT) board-to-board interface for stackable systems includes popular new interfaces such as PCI Express and USB. It also includes the Low Pin Count (LPC) bus to enable smooth, legacy-friendly migration. But creating an interface that enables older technologies only solves part of the problem.

SFF-SIG standards separate the interface specification from the board form factor specification. SUMIT is a pure interface standard that specifies connectors and pin definitions for a board-to-board interface. One obvious place to implement a SUMIT interface is on the ubiquitous stacking 90 mm x 96 mm form factor known as PC/104. Because no pure form factor standard for PC/104 existed, SFF-SIG created the Industry Standard Module (ISM) Specification (available at www.sff-sig.org).

But one problem still remained. Having separate interface and form factor standards allow the I/O stack to be placed anywhere on the SBC. However, when the SBC and I/O modules are the same size, as is the case with ISM, the location of the interface connectors must be specified. To meet this need, SFF-SIG created the new SUMIT-ISM Specification (also available at www.sff-sig.org) that describes how to implement the SUMIT interface on an ISM.

With the SUMIT connectors placed along one edge of ISM, the question arose if we should support a legacy connector – either PC/104 with ISA or PCI-104 with PCI – along the other edge. This stirred up a new debate: ISA versus PCI legacy support. We were far down the path to supporting the PC/104 connector when Jonathan Miller of Diamond Systems Corporation wrote an impassioned e-mail describing why he believed that PCI support was a better decision. The debate raged for weeks around the time of the Embedded Systems Conference in San Jose last April until the Diamond Systems folks came up with an even better idea – a way to support both the PC/104 and PCI-104 connectors with SUMIT.

The problem with supporting a legacy connector along the edge of ISM occurs because of the module’s asymmetrical mounting holes. Once SUMIT picked an edge, the other edge was predefined as PC/104 or PCI-104 by the mounting holes. Diamond Systems’ elegant solution was to slot the mounting holes on an ISM, allowing the opposite edge to support either a PC/104 or PCI-104 connector. (Actually, the slots are only required when the opposite edge is PCI-104.) This allows a SUMIT I/O stack to support either legacy PC/104 cards or legacy PCI-104 cards, but not both in the same stack. These are defined as Legacy Type 1 and Legacy Type 2 in the SUMIT-ISM Specification.

This is clearly a situation where a simple, logical, and elegant solution can provide incredible flexibility and legacy support to embedded designers. Whether the legacy bus support is provided on the CPU or derived by a bridge card in the stack (ISA from LPC, PCI from PCI Express), embedded designers using the new SUMIT interface now have a way to include a custom or unique PC/104 or PCI-104 card from their current system while embracing PCI Express and/or USB going forward.

During the past few weeks, SFF-SIG has also released the Pico-ITXe and Pico-I/O Specifications to the public, along with an update to the SUMIT Specification to v1.5, which incorporates USB channel shifting. More info about these specifications will be covered in the next issue.

Small Form Factor SIG 408-480-7900 info@sff-sig.org www.sff-sig.org