This is a short story of how ISA split apart the PC/104 industry. Here, all the hyperbole is distilled into a “Read this” primer that sorts out the various embedded board form factors.
I’ve written about the embedded boards industry for decades. At one point I even did some consulting for the PC/104 Consortium by recommending a focus on rugged and long-life applications and systems. But I can’t say I’m thoroughly familiar with all of the PC/104 specifications. There are just too darned many variations; who can keep them all straight?
Rest easy. Herein is a quick-and-dirty primer on all the specs, and how they compare. I’ve compiled this info courtesy of the PC/104 Consortium, the SFF-SIG, and friends from companies like WinSystems and Kontron.
PC/104 Consortium’s Specifications
I’m going to focus exclusively on PC/104-sized boards and ignore the related flavors like EPIC and EBX, but here’s how they look size-wise, compared to the original 90 x 96 mm (3.6 x 3.8 in) PC/104 board on the left:
A comparison of PC/104 board size to EPIC and EBX embedded boards.
PC/104 exclusively uses the ISA bus for stack-up and stack-down, whereas the other versions add or subtract PCI and PCI Express busses:
On a PC/104 board there are low-speed connections, all the way up to ISA, PCI, and PCI Express. This shows how the PC/104 Consortium’s line up adds I/O and stacks.
In February 2013, the PC/104 Consortium ratified and made public the PC/104-Express and PCIe/104 versions shown on the right. PCIe/104 is their board-of-the-future and comes in Type 1 and Type 2 versions, depending upon the peripherals and feature set needed in the system. The brand new PCIe/104 has provisions to support PCI Express Gen 2 and Gen 3. The primary differences are shown in green. Type 2 would be used for the highest speed peripherals such as USB 3.0 or SATA; however, connector pin limitations forced PCIe x16 onto Type 1 instead of Type 2:
The new PCIe/104 comes in Type 1 and Type 2 versions, depending upon I/O requirements.
Note that the legacy ISA bus, and eventually the PCI bus (in PCIe/104) are dropped as the industry moves to PCI Express. These older ISA and PCI busses are supported by adding bridge cards to the middle of a PC104xxx stack as shown:
Adding ISA or PCI to the newer PC/104 stacks requires a bridge module in the sandwich.
More information on stack-ups and how the PCI Express bus gets “lane shifted” as the stack grows can be found in the specifications for PCI/104-Express and PCIe/104.
Small-Form Factor SIG’s Specifications (SFF-SIG)
The industry fragmented over how to support the legacy ISA bus, and vendors that believed ISA I/O boards would remain popular for many years formed the SFF-SIG around 2008. Their PC/104-sized board is the same 90 x 96 mm (3.6 x 3.8 in) size but is called “Industry Standard Module” (ISM) to avoid copyright and trademark infringement issues. Instead, their specifications define Standard Unified Modular Interconnect Technology ISM boards (SUMIT-ISM) and the specification can be found here. An example of a larger EBX baseboard with SUMIT and PC/104 ISA connectors is shown below:
This is an EBX-sized baseboard that allows a SUMIT-ISM card to be stacked on it. The SUMIT-AB connectors are in the middle and the legacy PC/104 ISA bus connector is along the top edge. (Courtesy: WinSystems and TechBriefs.com .)
As for connectors and I/O on SUMIT-ISM boards, it uses the same Samtec Q2 double row, high speed 15.24 mm Q-strip connector system as does the PC/104 Consortium. The following table compares many of the common SUMIT-ISM I/O types (Column 1) to the PC/104 Consortium’s flavors, including the new Type 1 and Type 2 PCIe/104 just announced:
How PCI Express is implemented on SUMIT-ISM board and PCIe104 boards Type 1 and Type 2.
For additional explanation of how the ISA bus split the industry, read WinSystems’ article at TechBriefs.com .
It all comes down to a philosophical choice. If your design needs ISA and newer, contemporary processors, your choices are the original versions of PC/104 and SUMIT-ISM. When your system starts needing variations of PCI and PCI Express, you’ll need to examine how best to implement those busses in the stack-up: with or without bridge modules. If you just want PCIe, then both SUMIT-ISM and the new PCIe/104 modules have you covered.