本文主要讲解COM Express标准对于市场的引导，COM Express标准的意义及其有点，最后介绍COM Express标准的相关设计方面的细节。
1. COM Express标准
提到COM Express标准，这里不得不说明什么是COM标准。
1.1 什么是COM
所谓“COM”就是Computer-On-Module，即将一个Highly Integrated CPU及其最小系统做成一个Module，并且这个Module可以Plug into一个Customized Carrier Board上，从而构成一个产品或者系统，这里的Customized Carrier Board即用户定制的具有专门用途的主板，它包含用户需求的所有外部接口(SATA, PCIe, USB, Power, Audio, Video等，后续会有讲解)。这种架构具有如下两个特点：
<1>: 随着CPU的升级，我们只需要重新设计以新的CPU为核心的COM即可，而保留COM Carrier主板不变。
<2>: CPU保持不变，但系统的外部接口有变化，此时只需要重新设计COM Carrier Board即可，保留COM设计不变。
1.2 什么是COM Express
COM Express标准其实就是COM标准的扩展，增强版本。鉴于COM Express标准已经慢慢占据市场主流技术，本文主要讲解COM Express标准的相关内容。
1.3 COM Express与ETX的比较
首先介绍一下ETX，standing for Embedded Technology eXtended, is a highly integrated and compact (3.7 x 4.9 in.) (95 x 125 mm) computer-on-module (COM) form factor that can be used in a design application much like an integrated circuit component. Each ETX COM integrates core CPU and memory functionality, the common I/O of a PC/AT (serial, parallel, etc.), USB, audio, graphics, and Ethernet. All I/O signals as well as a full implementation of ISA and PCI buses are mapped to four high density, low profile connectors on the bottom side of the module.
àCOM Express是一个新的COM标准，它也叫ETXexpress标准，其实COM Express标准和ETX标准大部分是一致的，因此也会导致它们的混淆。COM Express is a new COM standard. It is also known by the name "ETXexpress"; somewhat confusing because this standard has actually nothing to do with the ETX standard.
àCOM Express是PCIMG开放的标准，ETX是一个私有的非开放的标准。
àETX标准起初是由一家公司制定的，它起初是基于以前的一些总线技术(即legacy technology)，但是随着技术的发展，特别是串行通信技术的发展，一些古老的总线技术慢慢遭到淘汰，因此ETX标准也需要一个升级版本，即COM Express标准应运而出。The ETX specification was based on the current technology of the time, but what is now considered legacy technology. Many of the legacy technologies are moving into the end of the product life cycle and may be disappearing altogether as well as they do not scale to the full potential of modern CPUs, GPUs and high-speed memory.
àCOM Express标准是基于最新的总线技术，适用于目前所以的高性能 CPU，同时它也提供传统总线技术到当今最新总线技术的过渡与兼容。The COM Express specification is based on the latest I/O technology, and can scale to realize the full potential of today’s CPUs, such as the Intel Core 2 Duo processors, GPUs, and high-speed memory. Because the COM Express specification allows legacy and new I/O technologies to coexist in the same design, it provides a safe migration path from legacy to new technology.
à随着CPU，GPU处理速度的加快，传统的ISA，PCI，AGP，ATAPI总线已经成为瓶颈，因此现在Serial ATA，PCI Express，Serial Attached SCSI (SAS)已经占据的主流市场。
àETX 2.0标准基于ISA，PCI，IDE，100MBase-TX技术，ETX不支持AGP接口，因此就无法外接GPU，ETX不支持1000MBase-T技术，ETX的PCB大小定义(95mm x 114mm)，PCB面积以无法满足现在双核或者多核CPU的应用，因此COM Express标准出现的时候，均克服了这些缺点。基于此，本文不会对ETX标准进行详细介绍，后续会集中介绍COM Express标准的应用及相关定义。
2. COM Express标准内容
该部分主要介绍COM Express标准的相关细节问题。COM Express is the PCI Industrial Computer Manufacturers Group’s (PICMG) new standard for COMs that defines a small mezzanine-style CPU module that is highly integrated with a set of high-speed serial interfaces。因此COM Express标准定义了COM及其COM Carrier Board之间的接口及其Connector，从而实现不同厂商的Module和不同厂商的COM Carrier Boards达到互连的目的，即it
ensures interoperability of modules and carrier boards from multiple vendors。
2.1 Form Factor
COM Express标准对于Module的大小定义了两种尺寸：
<1>: 一种是Basic Module。A basic module form factor that is 125 mm x 95 mm, roughly the same size as a small Serial ATA hard drive。
该种大小的Module主要是用于Mobile及一些空间受到限制的嵌入式系统，而在此条件下，内存通常use a single horizontal DDR or DDR2 SO-DIMM memory module。
<2>: 另一种是Extended Module。An extended module form factor that is 155 mm x110 mm, a little smaller than EBX single board computers。
该种大小的Module主要是用于OEM applications that need on-module space for bigger memory modules, chipsets, and processors，内存通常two full-size DIMM memory modules。
由上面两幅图可以看出，Spec不光定义了Module的大小，同时对Connector的精确位置也进行了定义。
2.2 Connector
COM Express的Connector如下图所示，ROW AB(220 Pins)和ROWCD(220 Pins)，其中ROW CD是可选的。
Spec为了以前的技术，以及以后的技术，标准定义了5种类型的Connector：Type 1 ~ Type 5。
Type 1：这个Connector提供了最简单，Low Cost的Solution，接口支持GbE, LPC, Serial ATA, USB 2.0, VGA, LVDS, optional TV, power management, GPIO, and other miscellaneous I/O.
Type 2: If we compare this to Type 1, the system gains an IDE port as well as PCI Express Graphics (1 x16) and PCI 32-bit/33 MHz
Type 3: The IDE that Type 2 gained drops out for two additional GbE ports in exchange.
Type 4 –Type 2 retains the IDE port, but Type 4 drops the PCI 32-bit/33 MHz. This configuration enables the addition of PCI Express lanes so that the modules can support 32 lanes.
Type 5 – This is the legacy-free pin-out type. It leaves out both the PCI and IDE interfaces leaving space for 32 PCI Express lanes and up to 3 GbE ports.
Under the COM Express standard, modules that follow Type 1 will support a single connector with two rows of pins – 220 pins total. Modules that follow Types 2 through 5 will support two connectors with four rows of pins – 440 pins total.
上面的示意图中只列出了一些典型接口，但实际上还存在着一些其他低速接口/管理接口等，如I2C，SMBus等(具体的Pin定义可参阅COM Express的Spec及其Design Guidelines：reference--PICMG_COM Express Carrier Design Guide包含各种接口的Design原则以及Layout原则.pdf)，这里以Type 2为例，其简单示意图如下：
2.3 Power Design
除了I/O requirements，connector placement 以及pin usage定义以外，Spec同时定义了Compute-On-Module的最大功耗，主电源12V供电，最大功耗188W(except for standby and RTC/CMOS backup functions)。从Connector的Pin Usage定义来看，系统中还有+5V的电源输入，spec对其描述如下：如果系统不使用standby function，则no +5 V standby input should be necessary。
2.4 散热设计
Spec还对Module的散热设计进行了定义。
2.5 COM Express的优点
使用COM Express技术设计嵌入式系统，目前已在业界形成共识，With a better understanding of what the COM Express standard is,evaluating the new standard’s contributions to the embedded computing community is more effective. The COM Express specification is an extension of the already widely accepted COM concept where a highly integrated CPU module plugs into a customized carrier board rather than a designer having to develop and build a
full custom motherboard. The benefits of COMs include:
<1>: Faster time to market
<2>: Cost-effective customization
<3>: Minimized risk
<4>: Lower product life-cycle costs
<5>: Lower total cost of ownership
2.6 兼容性
COM Express标准主要是以串行差分信号为主要技术基础(SATA-1.5Gbps，PCIe-2.5Gbps)，但其5种类型的Connector定义，使得其不仅兼容以前的总线技术(像IDE，PCI等)，同时也同样适用于未来的总线技术，即SATA-3.0Gbps，PCIe-5Gbps。Spec定义电气特性时均考虑了这一点。
