An I/O View of Small Form Factor Choices
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Technology General-purpose I/O options An I/O view of small form factor choices By Eric Rossi With all the small form factor choices available, the process of selecting an SBC takes more than a little time and consideration. Evaluating each option based on a handful of significant factors including I/O requirements helps designers make informed decisions. Designers looking for small form factor PC/104 connector. Although this form SBCs have many choices, including the factor has x86 roots, new PC/104 SBCs more widely fielded PC/104, EBX, and based on RISC (primarily ARM) archi- System-On-Module (SOM). With so tecture recently have been introduced. Figure 1 many options, how do designers deter- This allows RISC-based SBCs to make ly mine which one to use? use of the various PC/104 peripheral n modules. EBX The right choice depends on three key EBXO is a larger (5.75" x 8.0") format usu- issues plus several other considerations One of the definite advantages of thet ally utilized with higher-end processors, listed in Table 1: PC/104 format is the number of SBC and although low-end processor EBX boards peripheral options and enclosuresi navail- are available. EBX allows both PC/104 n What are the I/O requirements? able. The PC/104 standard allowsr these and PCI expansion. A standard PCI card n How much processing is required? peripheral modules toP be stacked one on slot provides for PCI bus connection (see n What are the constraints (power top of another so that almost any applica- Figure 2). Numerous manufacturers pro- consumption, size, temperature, and tion can bel implementede with a PC/104 vide boards with the EBX form factor so on)? module stack (see Figure 1). To keep up and mounting holes but do not abide by with theg higher PCI-based I/O bus speed the designated connector locations or in The answers to these questions will inthe PC/104 standard was expanded to some cases, eliminate the PC/104 con- provide the criteria needed to make an include PCI bus connections. This up- nectors altogether. This normally is not an informed choice. Let’s examine some S of dated specification, called PC/104-Plus, issue unless a designer is trying to replace these issues for each form factorr with a has a derivative called PCI-104 that omits a true EBX board or needs PC/104 when focus on how I/O can beo added. the ISA bus. it is not provided. Some manufacturers such as EMAC have enhanced their EBX PC/104 F Due to the popularity of the x86 archi- offerings by providing PC/104-Plus and PC/104 is a small (3.55" x 3.78") expand- tecture, the PC/104 format will run just Mini PCI. able format. Due to this size, PC/104 SBCs about any Operating System (OS) with usually are based on lower-end 32-bit x86 any programming language. Some of the processors (386 to Pentium). Some manu- x86 PC/104 options draw less power than facturers take liberties with the specifica- other larger form factors with higher-end tion and add small wings, increasing the processors, however RISC-based PC/104 size of the board and thus allowing faster boards can offer even lower power con- processors and/or more I/O. sumption. A PC/104 peripheral module will cost quite a bit more than an equiva- Derived from the legacy PC x86 archi- lent commodity ISA/PCI card, but the tecture, the PC/104 standard provides PC/104 card is a much smaller and more ISA bus connectivity through a special rugged alternative. Important considerations when selecting an SBC Is it a new application or a retrofit? What is the target cost? Figure 2 What is the development budget? Having a PCI card slot can be handy What Operating System (OS) and programming language are being used? when a specialty card that does not come Is there existing software that will be reused? in any other format is required. If more Will the system run on a battery or have any other special power supply requirements? than a single PCI slot is required, a riser card can provide two PCI slots that fold Is there a special card/module that must be used? over the EBX board at a right angle. The Table 1 problem with using PCI cards in this way PC104 and Small Form Factors ©2007 OpenSystems Publishing. Not for Distribution. Technology General-purpose I/O options is that they are difficult to mount and hold such as data acquisition and control, for connections makes an inherently more secure without fabricating custom brack- example, will require one or more PC/104 reliable system. ets. EMAC offers enclosures that handle expansion modules and possibly a termi- EBX boards with PCI cards holding them nal board or two. Although this is a less Like SBCs, SOMs come in a variety of securely. than optimum solution, it is off the shelf, flavors, some of which are standards minimizing time to market and incurring based and others that are propriety EBX, like the PC/104 form factor, is minimal design cost. Alternatively, a full designs. ETX, COM Express, XTX, and based on x86 architecture and will run custom solution might be favorable but STX are examples of standard SOMs almost any OS with any programming would incur a large engineering design covered by the ETX Industrial Group language. In addition, since EBX is rela- effort. Additionally, the processor core (www.etx-ig.org), PICMG (www.picmg. tively large, four serial ports and four or can be the riskiest part of the design, org), the XTX Consortium (www.xtx- more USB ports are usually provided as especially with higher-end processors. standard.org), and the STX Consortium well as one or more Ethernet ports. As (www.stx-consortium.com), respectively. mentioned, some EBX boards provide SOM splits the difference between off- These standards are primarily based on Mini PCI expansion, which is ideal for the-shelf and full custom designs, taking 32-bit x86 architecture and feature a adding wireless 802.11 Ethernet. the best elements of both approaches. This combination of ISA, PCI, or PCI Express semicustom SOM approach comprises expansion buses.l yThe advantage of these The 3.5" Half EBX format is a very pop- two components: the processor module SOMs isn that a number of manufacturers ular derivative of the EBX form factor, and the carrier board. The off-the-shelf second source them. Proprietary SOMs, which as its name indicates is half the processor module contains the processor, whichO are usually smaller and less expen- size of a full EBX board. In addition to memory, real-time clock, Ethernet, serialt sive than their standard counterparts, its smaller size, the Half EBX board uti- ports, hard disk and/or flash disk inter- come in a variety of form factors and use lizes onboard connectors for its standard face, and other processor-specificin I/O. a mix of 8-, 16-, and 32-bit processors. I/O (such as serial DB9, RJ-45 Ethernet, The module plugs into a carrierr board, They are usually pin compatible within a USB, PS2 keyboard, and DB15 video), which provides all theP system connectors manufacturer’s product line but not from eliminating some of the cables EBX and and any additional I/O components manufacturer to manufacturer even when PC/104 boards require. The cables are required forl thee application. the exact same form factor is used. usually not included with a full EBX board and are purchased separately. Whileg carrier boards can be purchased In either case, by utilizing a pin compati- inoff the shelf, enabling immediate soft- ble SOM product line, the SOM approach ware development, the carrier board must provides an upgrade path when future Sbe custom designed for the application enhancements call for a more powerful r at hand to reap the benefits of the SOM processor. In addition, once an applica- approach (see Figure 3). The customer tion is built around a particular processor “SOM splits theo or module manufacturer can perform a module, the development environment F custom carrier board design, but regard- and all the code written for it should less of who performs it, the time frame, come across seamlessly to the next proj- difference between design cost, and risk are reduced over ect. SOM also increases product longev- a full custom design. An SOM design ity. Unlike what’s possible with a full can decrease the cabling and expansion custom approach, the processor module off-the-shelf and full module stacking found in a strictly off- can be replaced easily once the processor, the-shelf approach, not to mention the flash, or RAM becomes obsolete. system unit cost savings. In addition, the custom designs, taking reduction in cabling and module inter- Narrowing the choices PC/104 is an ideal solution for applica- tions that demand an off-the-shelf small the best elements of form factor with low- to mid-range pro- cessing power. The primary disadvan- tages are multiple cables that tend to both approaches.” come loose if not secured and relatively expensive system cost when several mod- ules are required. EBX is well suited for high-end appli- SOM cations especially when a PCI card is While PC/104 and EBX have been around required. It has the same cabling issues as for a while, the System-On-Module PC/104 and is much larger than a PC/104 (SOM, also referred to as Computer-On- module.