Kishan Jainandunsing
(February 2006)

In this column we highlight monthly in a series of articles the value propositions of the COM Express standard. This month’s article is about pros and cons of the COM Express approach vs. the traditional SBC approach.

COM Express SBCs versus Traditional SBCs

Summary

The traditional single board computer (SBC) approach to embedded systems design has several drawbacks that limit its usefulness. In contrast, the COM Express approach does not suffer from these drawbacks. Its component nature provides embedded systems designers with the same flexibility as IC components - yet, without the long and costly development cycles that are common for embedded host computer systems that are designed from the ground up.

Traditional SBC Form Factors

The single board computer industry has created a plethora of form factor definitions for single board computers. The table below provides an overview of the most prevalent form factors, together with their methods of I/O breakout and functional expansion.

SBC	Size (mm/inches)	I/O Break Out	Expansion
EmbATX	243.8 x 243.8/9.6 x 9.6	Edge connectors	PCI/ISA slots
Mini-ITX	170 x 170/6.7 x 6.7	Edge connectors	PCI slot
3.5” ESBC	145 x 102/5.7 x 4.0	Headers	PCI slot
5.25” ESBC	203 x 146/8.0 x 5.8	Headers	PCI slot
PISA	155.5 x 125/6.1 x 4.9 338.5 x 125/13.3 x 3.6	Face plate connectors	Back plane PCI/ISA slots
PC/104+	96.5 x 91.4/3.8 x 3.6	Edge connectors & headers	PC/104 ISA and PCI-104 PCI headers
EBX	146.1 x 203.2/5.8 x 8.0	Edge connectors & headers	PC/104 ISA and PCI-104 PCI headers
EPIC	165 x 115/6.5 x 4.5	Edge connectors & headers	PC/104 ISA and PCI-104 PCI headers

Table 1. Most prevalent SBC form factors

Figures 1a through 1g show some real-world examples of these form factors, illustrating the different I/O breakout and expansion methods.

Figure 1. I/O break out and expansion for various industry standard SBC solutions

Pros and Cons of Traditional SBC Solutions

The SBC solutions of Table 1 have certain advantages and disadvantages as listed in the table below.

PROS	CONS
No motherboard design required. Ready off-the-shelf product. Standardized expansion slots - ISA, PCI or both. Large number of suppliers to choose from. Several form factors to choose from. Relatively low cost. No motherboard design required.	In general: Limitations in form factors. Primarily targeted for floor installed or bench-top designs. Bulky form factors through ISA and PCI add-on card expansion. Complex, space consuming and expensive cabling solution for interfaces. Non-scaleable chipsets limits product life cycles. PC/104+, EBX & EPIC specific: Large z-dimension due to stack-up expansion.

Table 2. Pros and Cons of Traditional SBC solutions

The limitations in form factors of the traditional SBCs means that handheld devices are impossible and portable devices are impractical when implemented with a traditional SBC. In fact, because the traditional SBC products target primarily non-mobile applications, power supply design is mostly aimed at an ATX power source with associated bulky connectors and wires.

Traditional SBC solutions are bulky, especially because of add-on card expansion. This prevents or severely limits practical enclosure designs. Further contributing to the bulkiness is the wiring that is usually necessary to route I/O to practical locations on the enclosure (Note: Solutions based on the PC/104+, EBX and EPIC specifications can be especially bulky because of the staking of PC/104+ mezzanine expansion boards). This wiring can be quite complex and significantly drive up the cost of the equipment, while negatively impacting the reliability of the assembled product. To aggravate matters, header locations more often differ than agree between products of different manufacturers and even between products of the same manufacturer.

An SBC product does not guarantee that its replacement will have the connectors in the same locations. In fact, connector locations more often differ than agree between products ofdifferent manufacturers and even between products of the same manufacturer.

Enter the COM Express Approach

In the COM Express approach the SBC solution consists of a COM Express module and a carrier board. This is illustrated in the figure below. The COM Express module incorporates all necessary components for a bootable host subsystem - and most commonly integrated I/O found in chipsets today (graphics, Ethernet, SATA/SAS, USB, etc.).

Figure 2. The COM Express approach to a SBC solution

The COM Express carrier board provides power to the COM Express module, brings out the I/O, defined on the module, to connectors, sockets and/or headers on the carrier board and integrates I/O and other functions not defined as part of a COM Express module. These I/O and functions are typically application specific, but can be generic as well.

The advantages and disadvantages of the COM Express carrier board approach are listed in Table 3.

PROS

CONS

Exact form & functionality fit. Equally suitable for implementation of floor installed, bench top, mobile and handheld devices. Lower cost carrier board due to PCB layer reduction. Design reuse within a single product line and across different product lines. Easily scalable in terms of performance and product life. Simple and low cost system assembly through custom fit. Compatibility with existing enclosures through custom fit. Relatively low development cost and fast time-to-market. Relatively low total cost of ownership (TCO) due to long product lifecycles.

Some design work involved for custom fitting. Marginally increased z-axis profile due to mezzanine structure. Marginally increased cost due to mezzanine connectors, but which is compensated by lower carrier board PCB cost.

Table 3. Pros and cons of the COM Express carrier board approach

The comparison in cost between a COM Express SBC-based system and a traditional SBC-based system has to be done taking into consideration the following cost factors beyond PCB layer-count related costs:

• The total cost of ownership (TCO). The main portion of the TCO is the cost of redesigning the enclosure or part of it to obtain a perfect fit when the SBC has to be replaced due to its end of life. A COM Express SBC does not need an enclosure redesign.
• The amortization of non-recurrent engineering (NRE) costs over the product's life. A COM Express SBC solution has a longer product lifecycle and therefore amortization of NRE costs can be spread over a longer time period to lower the ASP of the product from the get-go.
• The cost of inventory. A COM Express carrier board with similar functions as an SBC is cheaper as it does not contain CPU chipsets and other circuitry specific to the host subsystem. Inventory cost can be controlled separately for COM Express modules and COM Express carrier boards.

In many cases t hese additional cost advantages of the COM Express SBC-based approach may far outweigh the cost advantage of PCB layer count.

Application-Specific COM Express Carrier Boards

Some examples of application-specific carrier boards are presented below. These are chosen in two extreme categories - floor installed and hand held - to illustrate the versatility of the COM Express module approach.

Kiosks - A carrier board for a wide range of kiosk applications. This carrier board may have interfaces for one or more touch panels, one or more LVDS& DVI panels, flash card readers for various flash card formats (Secure Card, CompactFlash Card, Sony Memory Stick, etc.), Wi-Fi network communication, a printer, high-definition audio, a finger print sensor, a credit card reader, a USB or IEEE1394 camera.

Figure 3. Kiosk application-specific COM Express carrier board

Industrial tablet computers - A carrier board for a tablet computer intended for a wide range of industrial uses with different operational modes, such as statically mounted, mobile mounted (forklift, etc.), handheld, etc. This carrier board may have interfaces for a touch panel, an LVDS LCD panel, DVI-I flat panel and CRT display, PC Cards or ExpressCards, Wi-Fi network communication, high-definition audio, a finger print sensor, a USB or IEEE1394 camera.

Figure 4. Industrial tablet PC application-specific COM Express carrier board

Conclusions

The COM Express approach provides OEMs in the embedded industry with the ability to recoup product development investments over a longer period of time than the traditional SBC approach. Not only do OEMs gain better returns on their initial capital investments, but they can afford to introduce new products at lower prices as their NREs are spread over longer product lives and/or multiple product skus and/or product lines. In addition, these gains come with the flexibility to meet new software feature and performance demands, contrary to the traditional SBC approach. These new features and extra performance can be easily added by moving towards a more powerful COM Express module - one that uses current or next generation CPUs and chipsets. In short, COM Express is a boon for the embedded industry - a methodology and standard that has the true potential of revolutionizing the economics and innovative powers of embedded systems.

End of Part II