Special Small form factors face off XTX versus COM Express – the gloves come off By Colin McCracken Editor’s note: The XTX Consortium recently released the XTX 1.1 Specification, including complete definition of all signals, mechanical Kontron runs the ETX Consortium with description, and results from the comprehensive PCI Express signal path involvement from other stakeholders; simulation. The latest specification is available at www.xtx-standard.org. however there are no operating proce- dures, regular meetings, or bylaws. Visit The x86 Computer-on-Module (COM) suppliers are taking sides in the www.etx-ig.org for information about the absence of a unanimous decision for how to provide new I/O and bus specification. technologies to custom baseboards. In one corner – COM Express. In the other, a surprise challenger – XTX. Features, trade-offs, and industry Rules of engagement have changed heavyweights square off in a 10-round battle for module supremacy. Serial ATA (SATA), a lower pin count dif- ferential signal version of IDE featured COM products are entrenched in the not used in the application. There are no in new chipsets, has achieved faster data embedded computer board industry. unnecessary connectors; the baseboard is rates and lower cost-per-gigabit than leg- System OEMs have embraced COM custom. Custom circuitry can be added to acy parallel ATA hard drives. PCI Express solutions that combine the convenience whichever buses are brought to the base- graphics (referred to as a x16 or by-16 of an off-the-shelf CPU with the flexi- board. Figure 1 shows how SBCs bring I/O lane) have replaced AGP graphics on bility of a custom baseboard design. to cable-ready connectors, whether pin desktop and notebook motherboards. In This size and connector flexibility takes headers or molded PC-style connectors. addition, x1 lanes in the new chipsets COM architectures into applications that have twice the bandwidth of the parallel can’t be served efficiently by any fixed The reigning champion PCI bus. These advanced technologies form factor off-the-shelf Single Board Since 2001, the ETX form factor has beg for a new form factor in embedded Computer (SBC). been a multivendor standard. By now, applications. annual production volumes exceed half For x86-based COMs, there are three a million. With a substantial number of To confuse matters somewhat, Kontron has competing form factor standards: ETX, OEM baseboards in production, ETX is recently introduced proprietary ETX 3.0 XTX, and COM Express. As PCI Express the prevailing x86 COM standard. solution to address the lack of SATA sup- and Low Pin Count (LPC) buses replace port in ETX baseboard pin definitions. PCI and ISA, the stage is set for a battle Traditional interfaces are included in the The approach hangs SATA connectors off of supremacy between the newer XTX ETX connector pinout, including serial the side of the module rather than pass- and COM Express given that ETX does ports, parallel port, floppy, IDE (parallel ing the signals to the baseboard. It’s not not support either of these interfaces. ATA), PS/2 keyboard and mouse, and the clear at this time whether any other vendor OEMs are now trying to sort through the ISA bus. In addition, Ethernet, video, and will develop a similar product, posing a hype to determine how to architect their four USB ports are provided to the base- life-cycle concern for the first ETX 3.0 next-generation systems. board, as shown in Figure 2. product and for the specificationitself. COMs give I/O flexibility SBC COM products are relatively new to the 23-year-old x86 embedded board indus- try, first appearing in the late 1990s. Designs have exploded with the advent of industry standards such as the ETX format introduced by Jumptec (now Kontron) in 2001. Processors, chipsets, serial ports, Ethernet, video, power management, IDE, and floppy controllers are common between COM products and SBCs. However, the GbE COM concept is innovative in that I/O is brought to a custom carrier board or baseboard through high-density, surface- mount, board-to-board connectors. I/O can be routed to an OEM-selected connec- tor (molded or pin header) in a specific location on the baseboard or ignored if Figure 1 40 / Fall 2006 PC/104 and Small Form Factors Copyright 2006 In one corner, the heavyweight COM Express has roots in ETX, sharing ETX Module the same modular concept and originator. Kontron developed the specification, and RadiSys, PFU Systems, and Intel joined the effort later. Subsequently, PICMG formed a working group to adopt the specification. When the dust settled, the committee came up with two board sizes times five incompatible pinouts with many optional signals per pinout. To top it all off, the COM.0 specification calls out connectors not compatible with the huge installed base of ETX baseboards. COM Express features the aforemen- tioned high-end graphics, SATA, Gigabit Ethernet, and additional USB 2.0 ports. One of the five pinouts has a x16 lane and up to 16 more x1 lanes. Certainly, this gives the I/O bandwidth a heavyweight Baseboard appearance. Figure 2 There are two sizes of COM Express modules in PICMG’s COM.0 specifica- COM Express – Type 2 tion: 155 x 110 mm and 125 x 95 mm. Several vendors have introduced a third size, 95 x 95 mm, but at the moment there is no guarantee PICMG will pick up this new size. The process of releasing products ahead of published standards or updates creates some risk for OEM customers. The Type 2 pinout, popular so far, features a x16 PCI Express graphics interface to which a baseboard designer would attach an ATI or NVIDIA graphics chip, useful for high-end gaming or graphics display E applications. For most other embedded Gb applications, chipset-integrated graphics are sufficient. To observe how Type 2 of the five COM Express pinouts features PCI Express, PCI, and LPC expansion without legacy I/O, see Figure 3 and visit Baseboard the PICMG website at www.picmg.org. Figure 3 In the opposite corner, ETX XTX COM Express* the challenger Advantech, Ampro, and congatec devel- Full Legacy Legacy except ISA No Legacy oped XTX as a global standard to evolve PCI, ISA PCI, LPC, PCI Express LPC, PCI Express the ETX standard and fill the gap between ETX and COM Express. Customers seek- 10/100 Ethernet 10/100 Ethernet 10/100, GbE ing a logical and cost-effective upgrade (2) IDE (2) IDE, SATA SATA path from ETX to high-performance I/O (4) USB (6) USB (8) USB can choose the XTX form factor rather than reengineer their designs. Table 1 com- AC’97 AC’97 + High Def AC’97 + High Def pares XTX to ETX and COM Express. Existing connectors Existing connectors New connectors Existing board size Existing board size New board sizes XTX and ETX are open standards, mean- ing that multiple vendors collaborate in Legacy is defined here as the older PC peripherals and bus, such as ISA bus, RS-232 serial, IEEE 1284 the development, marketing, and main- parallel, floppy, and PS/2 keyboard and mouse. * Five incompatible pinouts make feature comparison difficult, so only those common to all five pinouts tenance of the specifications. Both are are included here. Otherwise interchangeability gets sacrificed. available free of charge, including royal- ties. However, ETX 3.0 contrasts previous Table 1 Copyright 2006 PC/104 and Small Form Factors Fall 2006 / 41 versions of the specification in that a license agreement must be signed to list XTX Module module products on the ETX Industrial Group website, www.etx-ig.com. COM Express is also an open standard, although the administration details differ somewhat. PICMG is a large organization with extensive resources, and users pay a modest fee to obtain the specification or to join PICMG. The XTX Alliance is a rapidly growing interest group of companies committed to promoting and using XTX in the embed- ded market. Visit the XTX Alliance web- site at www.xtx-standard.org. XTX maintains the same mechanical dimensions and connectors; thus, ETX solution developers can keep their existing carrier board and heat-spreader designs, Baseboard giving them a quick, simple, low-risk way to upgrade to state-of-the-art technolo- Figure 5 gies such as PCI Express, SATA, LPC, ExpressCard, high-definition audio, and One must ask what types of applications Alliance has emerged with a legacy- additional USB 2.0. One product example can use so much I/O bandwidth. friendly specification fit for the fight. is the Intel Core Duo-based XTX 830, shown in Figure 4. Promotional materials position the For technology insertions where ETX COM Express standard as not being baseboards are already designed, ETX and weighed down by interfaces that are not XTX are the clear choices. OEMs should part of new chipsets. On the other hand, not be tricked by brand names that include serial ports (UARTs) and other legacy I/O ETX but are really mechanically incom- are still used in many industrial, medical, patible with ETX. XTX provides a bridge and military devices; for those systems, to the future, a smooth transition without XTX is a more viable choice. sacrificing mechanicals, baseboard lay- outs, and power-supply designs. For brand COM Express modules use a different new designs, XTX and COM Express each connector from ETX that is not compat- have merits. Module suppliers are picking ible. Existing ETX users must redesign corners of the ring as the fight is shaping their baseboards to use COM Express. up to be a 10-round decision.