The Computer On Module (COM) concept has been around for many years. PC104 and ETX are two of the better known COM standards. The idea behind Computer On Modules has been to push the more complex design tasks in computer systems from the user to vendors that specialize in the design of CPU subsystems. Let the user of the COM focus on the value they provide - the design of the application and possibly a carrier board. Computer On Modules are popular in space constrained applications like kiosks, gaming, and other embedded applications where higher reliability, extended temperature, and 24 x 7 operation preclude the use of white box based PC solutions. With the use of COMs designers of systems no longer have to take on the challenge of designing high speed processor subsystems or developing system firmware. The memory, processor busses and common peripheral functions are all provided by the COM. Tasks like the creation and maintenance of the BIOS is also offloaded to the COM supplier. The user of Computer On Modules typically designs a Carrier based on a vendor's reference design. The Carrier provides a convenient way to breakout the I/O provided on the COM that is specific to the application.

The most recent Computer On Module standard is COM Express. This standard was developed by the PCI Industrial Computer Manufacturers Group (PICMG) - the same standards organization that developed CompactPCI, AdvancedTCA, Advanced Mezzanine Card, and a host of other standards. The goal behind Com Express was to provide a technology refresh to the existing COM standards. It added support for PCI Express, USB 2.0, SATA, gigabit Ethernet, increased processor power and improved thermal performance. COM Express is one of several PCI-Express based standards released by the PCI Industrial Computer Manufacturers Group (PICMG). One of the interesting things to note about the flurry of standards activities around PCI-Express is the common set of companies that developed these standards. The PICMG has released PCI-Express based transports for AdvancedTCA, Advanced Mezzanine Card, passive backplane, and is working on a CompactPCI version. The subcommittees that developed these standards have many overlapping contributors so that information can be shared between the subcommittees easily. This reduced the specification development time and allowed leveraging of simulation data reducing the total cost for the development of these standards

There are two module size definitions in Com Express - Basic (125 mm x 95mm) and Extended 155 mm x 110 mm). The Basic size is targeted at space constrained environments while the Extended sizes supports DIMM modules and higher performance processors. In addition to the two module sizes, Com Express defines five different pinout types. Type 1 defines a single 220 pin interconnect between the Module and Carrier. This interconnect supports 8 USB 2.0, and 4 SATA or SAS ports; 6 PCI-Express lanes; 2 ExpressCards; dual LVDS display channels; VGA; TV Out; AC97; Gigabit Ethernet; 8 GPIO pins; LPC interface; 8 GPIO pins and supports up to 120 Watts. Type 2 Modules add a second 220 pin interconnect. Type 2 Modules contain all the features of Type 1 Modules and add an 32 bit PCI interface; IDE; additional 16 PIC Express lanes (intended for graphics); SDVO and increases the maximum power to 188 Watts. Module Types 3, 4, and 5 redefine some of the I/O pins. Within each Module type there are optional and mandatory features. This can make the selection of a COM Express Module complex as you need to understand the pinout type used and the I/O supported. The Com Express specification tries to minimize the number of variants by specifically defining the features that are mandatory and optional for each pinout type and by defining a Feature Fill Order which provides guidance for the ports provided on the Module. An optional EEPROM locate on the Carrier can also be used by the Module to determine the compatibility of the set. Users need to pay attention to the Module type sued and the I/O payload provided.

One of the biggest challenges in designing embedded systems today is also one of the most often overlooked in standards - that is the thermal solution. As technology advances it is no longer sufficient to think of the thermal solution as an after thought in the design process. The Com Express specification takes this problem head on. The specification defines a heat spreader interface for the Module. The heat spreader couples the Module to the application specific thermal solution. By defining a standard thermal interface users of COM Express can change Modules without having to redesign the thermal solution. The Com Express specification also defines a clear set of terms that can be used to compare the thermal performance of vendor's Modules as well as specific thermal measurement criteria. The requirements for a common thermal interface and a common set of measurement criteria and thermal specifications make comparing the thermal performance of different modules a straight forward task.

ADLINK, a leader in the embedded computing market recently released the ETXp-IA533 COM Express Module. This Module can use a variety of Pentium-M processors from the low end Celeron class processor up to the 2.0 GHz Dothan "760" processor with 2MB L2 cache and a 533MHz processor bus. The ETXp-IA533 uses the 915GM PCI Express chipset and supports the new DDR2 533MHz dual channel memory interface. Peripherals support by the ETXp-IA533 include gigabit Ethernet; a x 16 PCI Express port for graphics; four x1 PCI Express lanes; a 32 bit PCI bus; SATA; USB 2.0; and LVDS/SDVO/TV-OUT. ADLINK also provides a Carrier (also referred to as a Base) that holds the Module and includes four PCI Express x1 slots; PCI Express x16 graphics/SDVO slot; 2 PCI slots; and a Compact FLASH slot. ADLINK will make the schematics and layout files of the Carrier available as a starting point for customers of the ETXp-IA533.