RTC Magazine

Small form factors are not a new invention, but the need to reduce size, weight and power has boosted their demand and resulted in several design considerations. PC/104  revolutionized modular industrial computers with its small size of 3.4” x 3.6” almost two decades ago.  However, since then the small form factor (SFF) module concept has multiplied into a barely manageable diversity of small factor boards and so called standards.

Generally SFF boards can be classified into three major groups:

• Single Board Computers (SBC) where all functionality is designed into a single board with no further extensions
• Stackable Computer Modules (de facto the PC/104 family), which can be configured from a bare SBC to large bus systems with arbitrary expansion options realized by standardized modules
• Computer-on-Modules (COM) that concentrate the main computing functions into a standardized module sitting on top of a standard or custom designed baseboard

Basic SFF Choices

SBCs provide the highest level of vertical integration and lowest production costs especially with higher volumes. If it is not possible to buy a baseboard off-the-shelf, it is necessary to create a custom board with all the needed functions. The effort to develop this takes a significant amount of time and qualified engineers due to the complexity of today’s super-fast and super-miniaturized chips—some with  .600 or .500 or even .400 mm pin pitches on the printed circuit board. 

Stackable computer modules, namely the PC/104 family, are beneficial because almost anything can be purchased off-the-shelf. This makes hardware development easy and decreases development time by requiring as little as planning, purchasing and stacking. So what are the downsides here? PC/104 systems are great for specific application segments that need something simple and robust as long as the demands for computing power, graphics performance and maximum thermal design power are not too high. However, the connectors and system overhead are relatively expensive, and most of the ecosystem available is still based on the ISA bus. 

Since technology has been moving away from ISA and parallel buses over the last two decades, the once uniform PC/104 world has fractured into too many subsets of variations and upgrades. Today there is PC/104, PC/104-Plus, PCI/104, PCI/104-Express, PCIe/104 and SUMIT-ISM. This often leads to a quite challenging mix-and-match within the ecosystem. What used to be an unbeatable benefit can become a severe burden now in new projects. Many of the traditional products and components have reached end-of-life and are difficult to replace. Cooling may also be a little tricky as thermal design power is limited. The current CPU generation Atom and AMD Gseries are the first, and often only choice for midrange fanless applications today. For some applications like panels, the cubical outline may be a problem and the question remains: Will PCIe/104 or SUMIT-ISM finally make the race, and will the ecosystem follow?

In terms of standards, COMs are a lot easier, but just four families prove to be actual standards and not simply logo-clubs. They are ETX, with its well established facelift XTX, and COM Express as the current absolute market leader for midrange- to high-end COMs. In addition, there is the relatively new Qseven  that is emerging as the first choice for low power, mobile and ultra-mobile COM applications. 

Shared COM Benefits

COMs are the easiest—and typically most efficient—way to outsource the “difficult part” of  the latest computer technology with its ultra-fine pin grids and highly EMC-sensitive high-speed signals as high-end computer technology is usually not a core competency of most companies. COMs come highly pre-integrated and ensure short development cycles. The split between general part (COM) and application-specific part (baseboard) enables easy-to-manage scaling and upgrading options. Switching to the latest computing technology with less power consumption and higher performance can be as easy as simply swapping the module. COMs themselves are universal and not bound to a specific application—they add up across companies and applications to large numbers resulting in lower prices. Higher numbers and richer diversity of applications also result in fewer design flaws and higher product quality of the COM.

All these advantages make COMs the most efficient way to keep pace with today’s trends toward higher complexity, smaller size, less weight and lower power consumption on the technical side, and cost reductions and shorter time-to-market on the commercial side (Table 1). Figure 1 shows a visual evolution and size comparison of these modules.

Figure 1
COM evolution from ETX to Qseven.

Table 1
Comparison of COM family features.

ETX, introduced in1998, was the first true standard for COMs. It comes with full legacy I/O and PS/2 support and has a size of 2.4” x 2.9” (95 x 114 mm²). ETX became an established standard in 2000, and due to its maximum possible TDP of 40 watts it has a wide installed base mainly in the industrial computing and automation market segment. Other industries with large installed bases of ETX COMs are medical, transportation and gaming. Together with PC/104, ETX is the most significant SFF standard with unlimited ISA support. Today ETX modules are mainly used in legacy projects with a need to support the more than twenty-year-old ISA bus. 

Typical applications with need for ISA support are industrial control and automation. Since the end-of-life of Intel’s 855 chipset, today’s ETX COMs are mainly featuring VIA, Geode or Atom Processors; some recent boards use AMD’s Gseries processors upgrading graphics performance to an actual level. 

XTX was introduced in 2005 and is a fully compatible facelift to ETX with native SATA support (4 Ports) and 4 PCIe lanes replacing the ISA bus on the fourth connector. This boosts I/O performance from 133 Mbytes/s to 2.5 G bits per PCIe lane enabling up-to-date high-speed interfaces on the baseboard. If ETX compatibility is required and no ISA bus is needed, XTX is first choice. It’s a low-effort upgrade path from ETX to today’s technology and high-performance processors beyond the 855 chipset as well as the gateway to dual core power. XTX can be used as a form factor compatible, smooth transition platform on the way from ISA and PCI technology to today’s high-speed serial buses (PCIe). The main market segments are the classical ETX segments of industrial computing and automation, medical, transportation and gaming.

COM Express was introduced as a standard by the PICMG in 2005. The main goal was to introduce a universal, scalable and legacy-free COM standard. Sizes initially specified by PICMG were 2.4” x 3.2”(95 x 125mm², basic) and 2.6” x 3.9”(110 x 155 mm², extended). A large group of COM Express manufacturers has agreed on an additional and most popular size of  2.4” x 2.4” (95 x 95 mm², compact), which is roughly the size of a PC/104 board (90 x 96 mm²). Apart from a new connector design and these multiple size variants, there are some significant new features: 6 PCIe Lanes, a PEG Port with another 16 PCIe Lanes, SDVO Interface, on-COM Gbit LAN and a change of the supply voltage from 5 to 12 volts. The maximum allowed specified TDP was also raised from 40 (ETX/XTX) to 188 watts (extended), enabling the most powerful high-end processors and graphics chip sets. Legacy-free, and pushed by the PICMG, COM Express is the most popular COM standard today and has the richest ecosystem and vendor support. There is a great choice of embedded x86 processors for COMs spanning from single-core low-power Atom to high-performance quad-core i7. 

Market segments are midrange/high-end gaming, midrange/high-end medical, midrange/high-end digital signage, industrial computing, automation, telecommunications, transportation and high-end POI/POS/kiosks. 

Qseven is the most recent COM standard (Figure 2). It was created to support small-sized, low-power, mobile and ultra-mobile applications. It measures just 1.6” x 1.8” (70 x 70 mm²) and does not require an expensive board-to-board connector but rather an inexpensive, but reliable, 230 pin edge connector using an MXM card slot as known from mobile Graphic cards. The TDP is limited to 12W, but even more important, the specified supply voltage is 5 volts so that a mobile device can run efficiently on 2 lithium cells. Qseven supports no legacy beyond 32-bit PCI and LPC and just 4 PCIe lanes and no PEG. Onboard graphics support 2 LVDS ports and SDVO (shared with HDMI/Display Port). New are CAN, SPI and Card SDIO interfaces to ensure maximum flexibility for mobile applications. Qseven is not an x86-only platform; ARM is also specified and supported with the latest version as well. A very useful new feature is a common software API for industrial applications like watchdog timer, I²C Bus, display brightness control, BIOS storage area and reading of system temperatures. This enables a safe and easy exchange of boards from different manufacturers without software adaptations.

Preferred Market segments for Qseven are handheld and ultra-mobile devices, Panel PCs, entry-level gaming, entry-level/midrange medical, entry-level digital signage, industrial computing, low-power industrial computing, automation, transportation, entry-level and mobile POI/POS/Kiosks, any kind of battery or PoE operated x86 computing device.

So How to Choose? 

The way to find your favourite SFF COM is easy. First check your legacy and compatibility needs. If you need ISA, go ETX. If you need ETX compatibility and no ISA, go XTX. An overview of the selection criteria for the major COM modules is given in Table 2.

If you start a new, legacy-free project it’s likely to choose COM express or Qseven for longevity and latest technology. If your TDP exceeds 12W, choose COM Express. If you need a PEG Port to use an external graphics card, choose COM Express. If you build a handheld, mobile or ultra-mobile device your choice is most likely Qseven. If you consider a non-x86 Platform, your choice is most likely Qseven. If you run on batteries or Power-over Ethernet (PoE), your choice is most likely Qseven. If (small) size matters, then your choice is most likely Qseven.

Finally, always keep in mind that when you decide to use a COM, you still need a baseboard. The best COM is worth nothing without good support from your COM vendor or distributor.  

congatec US
San Diego, CA.
(858) 457-2600.
[www.congatec.com].