Volunteering A Long-Term View

Mike Caddy looks at a PC board design strategy.

Mass-market PC motherboards have traditionally come with a caution for embedded applications because long-term availability of that particular board may not be guaranteed.

Some manufacturers like ITOX and Evalue offer 'build-level controlled' boards in the popular formats – ATX and Flex ATX, Mini ITX and LPX . These premium priced boards come with a guarantee of support.

But there is a new alternative that offers protection from the short-term vagaries of the volume PC motherboard market. This concept, known as the ETX processor module/system baseboard, is one that is increasingly supported by a number of the single board computer (SBC) and applications-computing specialists like Advantech, IBASE and Evalue.

A MODULAR MOTHERBOARD
The concept involves locating the CPU and immediate chipsets on a small ETX processor module that is plugged in to a master 'carrier' board that has the low-level logic and connectors. Effectively, the motherboard functionality is located on a daughterboard, leaving the motherboard as more or less a backplane.

ETX processor modules are available in a variety of CPU/ performance combinations from a number of different vendors, giving the designer multi-vendor support. The module (sometimes called a 'system-on-module') is manufactured according to the ETX standard, and is intended for industrial and application-specific computing, using processors for which support will be available for some years. As it is manufactured in much larger volumes than the carrier boards, there are economies of scale in the production of the module. The performance level can be decided now, or varied in the future, as faster processors and technologies emerge.

As the 'carrier' board doesn't have the CPU and memory silicon, it is very easy to design, and highly customisable. It can be a bespoke proprietary design, or can be one of an emerging number of standard-form boards designed to emulate ITX and other styles. If required, it can be designed with the functionality and connectors to implement a whole range of different end products.

This gives the designer the best of both worlds. A system board which can be plug compatible with an existing solution and configured to meet their needs, as well as the opportunity to take advantage of the spread of processor modules now available.

'Standard' carrier baseboards are emerging, with designs broadly compatible with some of the more established SBC formats, allowing users of SBCs to move to a 'system on module' plan. These include– 5.25in (also called 'biscuit'), half-card with ISA or PCI finger.

DESIGNING A DIGITAL ANALYSER
To illustrate this concept, let's take the example of a range of digital analyser products. These rely heavily on number-crunching and use embedded operating systems to achieve a robust operating environment. Let's consider a situation in which the main board used in the design for the past year is at the end of its life.

We have two options. A new board, which will have a different physical layout and will incur some inevitable re-design. Alternatively, we can make a one-time investment and design a baseboard that will apply to all the models in the range, and fit it with different system modules according to the processing required. This gives the opportunity to implement a range with a number of different performance levels. Each could have a different display option, and the system main board can remain consistent across the range.

Features such as flat-panel-display direct drive are also likely to be available, as the manufacturers of the baseboards will be able to include the display drive they build on to their SBCs as standard.

Even if the designer opts to emulate a main board design on their semi-custom baseboard, they can take advantage of multi-vendor support for the critical system module, and remain largely immune from changes provoked by CPU market fluctuations.

Having completed the system board, there are two parallel upgrade paths for the solution. For the processor module, there is security of supply of the existing module for customers, allied to the ability to take advantage of emerging new processor technologies without a major redesign where this is required. The baseboard remains the same (or small changes are made, and these can take as little as 30 days to reach prototypes) while the processor module is swapped for one with more horsepower.

At the same time, it is relatively simple to add further low-level systems electronics on the system board, such as touchscreen decoding, data acquisition, power conditioning or DC-DC conversion, display logic, backlight inverters and so on, giving a controllable upgrade path for the product line. Again, there is the option to continue to support past designs of system for customers wanting this, and offer upgraded versions where customers need the enhanced functionality.

A further advantage, especially in the sensitive defence sector, is that this approach allows us to protect any confidential elements in the design of the system board. This is already being done in the defence industry where freedom from obsolescence is expected, and vital.

CONCLUSION
Whilst this approach doesn't entirely avoid occasional re-testing and approvals stages, it could eliminate future re-tooling or major redesign during the planned life of the product. For some applications, the total stability of a premium design of build-controlled motherboard will still be essential. For others, the coherent and strategic product range evolution, increased commonality between sister or cousin products and freedom from unplanned or unexpected major component changes of the system module approach could be compelling.

See associated figure

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