Selecting a stacking-bus format

As the new Chair of the PC/104 Consortium Board of Directors, there’s a lot to take in. There has to be a focus on the future and where the Consortium is going as a standards organization. But as we look to the future we often forget where we are today. What’s available off-the-shelf for those of us that need to build a system now?

The important question when designing a system is not “Should I use PC/104?” The question is “Which PC/104 format should I use?” PC/104 is no longer a single specification, but a form factor that supports and combines a number of interfaces. Twenty years of PC/104 evolution has resulted in a wide range of performance and cost options, while preserving the advantages of the stacking architecture (flexibility, time-to-market, and cost).

Counting the options

Despite a somewhat confusing list of names for all of the formats, there are basically eight PC/104 formats to choose from. To make these simpler to discuss, let’s ignore the names and simply number them as PC/104 Types 1 through 8. For the purposes of this article they have been numbered in order of introduction, which puts them in general order of performance and technology development. The footprints of each format, and their names, are shown in Figure 1.

Where it started

Type 1, the original PC/104 format, was released in 1992. Using only the ISA bus interface, the board-to-board communication at 8 MHz it is quite slow by today’s standards. That being said, how much bus speed do you need to read a temperature four times per second, or turn a motor on every few minutes? There are many embedded applications that do not need a higher-speed bus interface.

Type 2, released in 1997, added the PCI interface, but remained backward compatible with Type 1 by including the ISA interface. At 33 MHz, the PCI interface is much faster than the ISA bus and proved useful for higher speed I/O devices. It is excellent for systems needing low or medium speed I/O expansion. Types 1 and 2 are still the workhorses of many PC/104 systems.

Figure 1: PC/104 connector footprints in the eight format types.
(Click graphic to zoom by 1.9x)

Type 3, released in 2001, includes just a PCI interface. Type 3 I/O boards can be “mixed and matched” in a stack with any other PC/104 format that includes the PCI connector, such as Types 2, 3, 5, and 7.

The next generation

Types 4 through 8 represent a technology jump. They all feature some type of PCI Express interface running at 250 MB/s per channel. This allows them to interface with other boards in the stack many times faster than their predecessors. These formats also make a departure from previous iterations in that the interface connector does not support a single bus, but combines a number of high- and low-speed signals and busses. This increases the flexibility and lowers the cost of expansion cards, as they can tap into the type of signal needed. High-speed signals are available for intense processing, like video data, while lower speed interfaces are available for simple I/O, like additional serial ports.

Type 4, released in 2008, uses the SUMIT interface (SUMIT is a trademark of the Small Form Factor Special Interest Group – SFF-SIG). The connector includes PCIe x1, PCIe x4, USB, LPC, SPI, and SMB channels. The optional ISA connector allows support of both Type 1 boards and higher-speed Type 4 boards.

Types 5 and 6, released in 2008, focus on very high performance applications. The connector includes a PCIe x16 lane, which can transfer data at very high speeds (250 MB/s x 16). Types 5 and 6 include PCIe x1, PCIe x4, PCIe x16, USB, and SMB.

Types 7 and 8, released in 2011, use the same physical connector as Types 5 and 6, but changed a few of the signals, primarily to bring SATA signals up through the stack. They include PCIe x1, PCIe x4, SATA, USB, LPC, and SMB. Due to a common pinout for most of the signals, most I/O board designs will be compatible with both Types 5/6 and 7/8.

Choosing a format

What’s the next step in making a selection? Determining which processor and I/O functions are available on a given format. The website can help. The Consortium’s Products page searches a database of products across 60+ PC/104 manufactures. Hundreds of PC/104 CPU and I/O expansion boards are currently available. Developers can also search for products at PC/104 and Small Form Factors’ database: Finding an off-the-shelf solution, even for complex requirements, could turn out to be the easiest part of developing a new system.

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