When designing a bespoke embedded PC system which involves custom electronics and I/O connections there is the question of whether a standard SBC (Single Board Computer) should be used or a Computer-on-Module based solution (Com Express, QSeven, ETX etc.).
Both methods allow the computing element of the design to be bought in as a ready made assembly allowing the custom electronics to be designed and interfaced to the computer board.
With an SBC, interfacing to the custom electronics is generally through USB, COM Port, Ethernet or GPIO (General Purpose I/O) although it is possible to interface via PCI Express if the SBC makes this available with a full slot or PCI Express Minicard socket. With a Computer-on-Module the interfacing is generally through the PCI Express interface, although the above mentioned options for SBC are also available.
Why adopt the computer-on-module design?
One reason for adopting the Computer-on-Module design is the perception that it can easily be upgraded in the future by “simply dropping in an upgraded module” but this is rarely the case. Most Computer-on-Module applications require customisation of the BIOS and cooling features. Some designs use Computer-on-Module features that are unique to particular modules which locks the design into one manufacturer. Even with the same manufacturer unique features are not always migrated forward to newer versions.
Using the SBC in a design makes it easier to change the computing element but introduces other issues such as the more difficult mechanical mounting arrangements and more unusual interfacing requirements that must be addressed. It may be easier to design a carrier PCB with custom electronics and I/O with a Computer-on-Module than to develop unusual mounting arrangements and connectivity.
There are a wide variety of module shapes and sizes – COM Express has many sizes: mini; compact; basic and extended. It also has a number of different I/O combinations: Type 1 through to 7 and Type 10. QSeven offers a standard size or micro size version but with the same I/O combination.
Likewise SBC offerings come in a large variety of shapes and sizes: Pico-ITX, 3.5”, Mini-ITX, 5.25”, Micro-ATX etc.. There is no standard for I/O connectivity on the boards and each must be selected on a project by project basis. However it is generally easier to drop upgraded versions into the design.
The choice of approach to the design will depend on the application and the environment it is being used in. It is not always clear cut however and there are very successful designs using Single Board Computers in quite harsh environments with quite high shock and vibration requirements. Here you would have expected the Computer-on-Module solution to prevail but this is not always the case.
At BVM we have a broad range of both modules and SBC and are able to provide advice based on experience. We also have electronic and system designers available to assist your designs.
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