The organization responsible for the PC/104™ Specification has decided that new CPUs with PCI Express expansion should not also have ISA expansion. This is a serious
blow to the 70+% of the market who still use one or more ISA I/O cards in their stack. This mirrors the desktop motherboard mentality of migrating to PCI Express and PCI slots together. It also ignores
the installed base of PC/104 ISA users and the tight coupling of application software to these I/O cards. Rather than forcing the vast I/O ecosystem over to PCI, PCI Express or USB interfaces,
the SFF-SIG is offering an alternative - replacing the parallel PCI Bus with SUMIT while retaining the ISA Bus (see the illustration above). PCI Express is software-compatible with PCI, and PCI chips are rapidly vanishing from the market.
The first generation stackable bus is known as 'PC/104' (ISA Bus) which dates back to 1992 as an openly-governed standard. The second generation bus is known as 'PC/104-Plus™' which added a PCI connector to the opposite
end of the board. There was division among the 5 remaining stackable x86 CPU/SBC manufacturers located in the US about which bus connector to replace once
PCI Express came along - ISA versus PCI. The choice was necessary due to insertion registration limitations of high-density high-speed connectors (for Express).
In the end, four US manufacturers decided on SUMIT™ to retain ISA while the fifth created PC/104-Express for the PC/104 Consortium.
The product photo above to the right shows a SUMIT-AB implementation (top edge) with PC/104 ISA Bus (bottom edge). PC/104 I/O cards like the one shown to the right can plug directly into the CPU/SBC
without the need for a PCI-ISA bridge card since ISA is supported directly. Any PC/104-Plus (PCI) I/O cards such as Ethernet can be replaced by a SUMIT-ISM™ Ethernet card (NIC) which
passes through the PC/104 Bus. Note the side benefit of extending the lifecycle since the PCI devices are mostly EOL already.
After all of the SUMIT™ cards are stacked, then ISA I/O cards are added on top. Although very few legacy stackable applications need the speed of PCI Express, rapid
obsolescence of PCI endpoint devices (ICs) combined with the disappearance of parallel PCI from the new 2011 chipsets are forcing this transition. Luckily, this change is transparent to application software.
In order to learn more and see the benefits of SUMIT™ as the clear choice for the 3rd Generation stackable architecture,
to download a detailed comparison of the two stackable express architectures - SUMIT™ and PC/104-Express™.