Fastwel: StackPC form-factor – a new approach to embedded modules and systems development

Version 1

    Part 1. Protected computers based on Single-board, Stack- and COM-modules


    Alexey Sorokin (Moscow)


    Protected computers based on three types of embedded items – single-board computers, COM- and stack modules – as well as method of protected equipment development on their base are considered. PC/104 and StackPC specifications are described in brief.


    Protected computers and methods of their development

    Computer equipment manufacturers offer a variety of solution for all scientific and technical areas. There is a tremendous choice of components, modules and ready-made systems. Each item, whether it is a module component or a system component, is characterized by its stage of completion and application field.

    When a developer gets a task of integrating a new solution into a newly developed or upgraded system, sufficient time has to be spent for component base selection considering design specification requirements. A solution of this task in most cases can be narrowed down to two options: new system development or existing system modification. As a result, a system adapted to a certain task shall be obtained. Adaptation in this case can be understood as new item development using an existing component base according to a separate design specifications, or existing item upgrade according to new requirements without considerable rework of the item.

    The main difference between new system development and adaptation is a degree of versatility of the resulting product. If a new item is developed for a certain task, we obtain a specialized device, probably, with some redundant functionality for the future. This device can fit ideally into the system since it has nothing excessive and versatile, in a general sense, and the device form-factor and components location are defined by the certain system. But manufacturer incurs additional expenses while developing such product. First, comprehensive laboratory, bed, determinative and acceptance tests, as well as product release to manufacturing are needed. Second, seriation of this product and, probably, of its components will be less than of the manufactured analogues. Therefore, cost of a new model development and launch is always higher than cost of adaptation and modified product manufacturing. Therefore new devices are much more expensive than popular serial products.

    But if a product is popular, its price decreases significantly over time. Customers, as always, need high-quality products at possibly lower price, and manufacturers think of profitability: development of successful products and their modifications is much more profitable. Therefore adaptation gets more and more attention, and nearly each new product for an open market has vast modification potential requiring no significant time and labor spent.

    Possibility of products adaptation, on protected computers example, depends on the following factors: applied modules standardization, their producibility, wiring layout complexity inside the product, as well as mechanical components producibility – case, heat sink, modules fixture etc. Depending on problems being solved, additional reliability requirements may be applied to the product – from electric and mechanical parameters to electromagnetic compatibility, mean time between failures, body, connectors and cables protection degree. These requirements affect component base selection and product body design. For example, such concept as “protected”, using common IPxx (Ingress Protection Rating) classification system, is hard to expand onto a module or a set of modules not enclosed in a body. However, for a system consisting of modules set, a body or a frame, the protection is an essential characteristic.

    The IP65 requirement specifies watertight connectors and interface cabling inside the body. Use of a single-board computer will lead to bulky cable routing because mating parts of standard connectors are far not miniature. Conductive heat dissipation from a watertight body shall also betaken care of. It is not standardized for a single-board computer which is equipped, as a rule, with a ribbed heat sink, and not with a plate conducting heat onto body. Therefore requirements for protected computer body will most probably lead to selecting modules of such popular form-factors as COM or ETX (with carrier board development), PC/104-plus or PC/104-express (with peripheral modules set) (see fig.1). Other versions are possible, of course, but they are more expensive and therefore less popular.


    Each manufacturer finds his own answers the questions of modules conformity to certain standards. Developers often have to use various methods of similar systems construction while working with modules of the same standard due to the only reason that similar units on two products conforming to the same standard are executed in different ways. This may be caused by deviations from a standard or, as opposed to that by lack of standard requirements, for example, for main components and units positions.

    It shall be understood that any standard can describe only key moments common for all modules. The more such moments are distinguished and described in standards, the better compatibility of modules from various manufacturers. However excessive standard itemization will restrict manufacturers.


    Embedded single-board computer based solutions

    There are three module types closely interrelated to each other – these are single-board computers (SBC) with an option of functional enhancement, COM-modules (Computer on Module) and stack modules. Although the exterior of these modules differs significantly (see fig.2), they realize common approach to systems building called stack approach.


    For example, COM-module is mated with a carrier board, to which, in turn, modules of another form-factor can be connected. Stack modules of PC/104 family are mated to each other without any coupling boards, organizing easily configurable and, at the same time, rigid and standardized structure. SBC are, in fact, fully self-contained modules of standardized or arbitrary form-factor and usually, are manufactured in one of the most popular form-factors – EPIC, EBX or 3,5”. AT that, if we talk about extensible SBC, we mean the standardized method – installation of additional stack modules – as the most suitable option for embedded systems.

    An embedded SBC nearly always has interfaces or buses for functional enhancement by means of stack peripheral modules.


    COM-modules based embedded solutions

    СОМ-modules are not used separately, carrier boards are usually developed for them. These boards often contain additional interfaces and enhancement buses, as well as SBC. COM-module installed onto a carrier board is actually a SBC with a replaceable CPU module, because a carrier board and a CPU module represent an integrated system. A host COM-module shall provide not only mechanical compatibility with the carrier board but also support the whole functionality of the board. Therefore BIOS software for COM-modules is debugged by manufacturers for certain carrier boards or provides recommended solutions for carrier board developers for certain COM-modules.

    There are several types of COM-modules – these are products made to specifications COM Express (PICMG COM.0), ETX/nanoETX and COM-Express mini (Kontron). Ore specifications of “COM-like” solutions such as CoreExpress, COMIT (SFF-SIG), FCOM, StackPC (Fastwel) are less popular (see fig.3).


    Printed circuit boards (PCB) of CPU modules can have up to 12 layers, and components hold up to 90% of their area. CPU module manufacturers are fully responsible for production and assembly quality control. In case of carrier board most components are pin-type interface connectors, capacitors, resistors, digital logic circuitry components and components for interfaces protection. It is much easier to design carrier board with required functionality and support already designed CPU module than to design a new SBC with a little different functionality. Additionally carrier board can be expand with stackable expansion module.


    Embedded solutions based on modules of stack form-factors

    There are two basic types of expandable systems — bus-modular using backplanes and stack-based, where modules are connected to each other. Any other types are combinations of these two. A good examples of bus-modular systems are products in CompactPCI, VME and MicroPC form-factors. COM-modules may be referred as stack modules since they are joined to carrier boards as well as stack modules. The only difference is that COM-modules and carrier boards belong to different form-factors. The most popular stack solution is PC/104.

    Stack modules use piggy-back connectors as extension buses, like in PC/104 and PC/104-plus, or two different-type surface mount connectors at two sides of the board, like in PC/104-express. Connectors are matched so as mutual orientation of modules was preserved during their mating. These connectors are called Expansion Bus Connectors, distinguishing TOP and BOTTOM connectors. Connector pin assignment is described in the specification. Depending on functionality type claimed by the manufacturer, various signal circuits are used, at that specification describes the order of their use. For example, peripheral stack module PC/104-express must use the first channel PCI-express from the extension connector. Use of the second channel is impermissible if the first channel is not used since it will cause the whole system operation breakdown.

    Significant difference between stack modules, SBC and COM-modules is in the type of interface connectors used and in principles of its interfaces use. Only pin mating for interface connectors are used on stack modules, and nearly never – standard ones for personal computers. The reason is that stack modules are assembly units for various systems, and interfaces are lead out via specialized connectors specific for the certain solution. Maximum possible number of standard-type connectors is used for SBC that precondition their application field – non-watertight BOX PC. COM-modules contain no interface connectors at all since their interfaces are lead out to extension bus connectors and designed for further use on the carrier board.

    As it was noted above, PC/104 is the most popular stack systems family. Attempts were made to arrange new stack form-factors, for example, SUMIT (SFF-SIG) and StackPC (Fastwel). It could be noted that far not all modules in PC/104-express standard are compatible to each other, and this specification contains no recommendations on system design, but only few common examples of modules connection to each other. Therefore, the following problems appear again and again while working with PC/104: how to arrange a stack of CPU module, peripheral modules, power supply and an interface module with interface connectors; how to arrange waterproof and protected system efficiently, with effective heat dissipation; how to place interface cables inside the case etc. Specification StackPC appeared as a result of analysis and hardware elaboration of these questions. It is designed to answer the most of the above questions and describe not only modules but the whole system as well.

    StackPC specification defines three basic requirements:

    1. 1) Extension bus connectors pin assignment and mutual position;
    2. 2) System arrangement based on StackPC and PC/104-plus modules, as well as general requirements on StackPC approach adaptation to other popular form-factors.
    3. 3) New opportunity for stack modules – use of StackPC modules for COM-applications. This application was inacceptable for PC/104 form-factor products previously.

    All these aspects of StackPC specification will be considered in the second part and compared to PC/104-express specification.


    About Fastwel

    Fastwel for more than 15 years has been one of the leading companies developing and manufacturing rugged Single Board Computers for mission-critical applications in transport, military and defense, security and telecom. We deliver CPU modules for aerospace equipment, industrial automation and process control systems. Besides standard products manufacturing, Fastwel offers a full range of OEM and ODM services. To learn more, visit .