Electrical/Electronics

MicroTCA is a new specification that offers very high performance packed in a small form factor. The new specification is expected to be used in a wide variety of applications, including mil/aero, telecom edge, medical, enterprise and data, and scientific applications. However, there are so many possible configurations, it can be overwhelming. How can one develop various systems and offerings without starting from scratch — and the time to market, high costs, and implementation issues this brings? One solution is using modularity in MicroTCA designs. Prototyping and development of a new system enclosure design can be a time-consuming and costly process. Building upon a proven modular platform allows a wide range of design options with significantly reduced effort.

Options Aplenty

The MicroTCA architecture will allow large arrays of AdvancedMC modules to be used in a wide range of applications where a lower-cost solution is required than could be achieved by the standard AdvancedTCA architecture. The MicroTCA backplane allows single or redundant virtual carriers to provide power management, platform management, and fabric connections to a greater number of modules than a single physical carrier card could support in a classic ATCA application. MicroTCA systems will support up to 56 single-module/ full-size AdvancedMCs in a 19” EIA rack or an assortment of double modules either compact, mid-size, or fullsize. AdvancedMC modules are targeted for such modular applications as storage arrays, firewalls, blade servers, and even home entertainment centers. Each module may dissipate between 20 and 80 watts and the platform management scheme is designed to support applications from 99.99% to 99.999% availability. Assuming 3.125 GB/s data rates, the performance of a MicroTCA system slotto- slot is 6,250 MB/s.

MicroTCA can come in a wide number of configurations, including single (75-mm high) or double (150-mm high) modules, in compact, mid-, or full-size. Further, pico, cube, or subrack chassis formats are all possibilities (see Figure 1). The MicroTCA standard was developed with stamped sheet metal enclosures in mind, which is attractive for high volumes, but what about new product development, prototyping, and small-to-medium volumes? The strength of MicroTCA is its flexibility, and that flexibility requires the designer to make a lot of choices. Depending on his exact design criteria, it could be difficult to build and optimize a backplane design without the time and expense of heavy customization. All sheet metal parts need to be drawn and these parts are quite complex. Furthermore, a flat pattern and a punching program have to be created. Modular enclosures designed specifically for MicroTCA can solve this problem.

Modular MicroTCA

The idea of modularity in embedded chassis is to be able to change size (height, width, depth) without the need for a lot of design or custom machining. A well-proven approach is replacing the punched card cage with aluminum extrusions and injection-molded plastic card guides. Extrusions are similar in concept to erector sets that you may have played with in your youth. They have holes cut at regular intervals to accept card guides in whatever location you choose. They can be easily cut in different lengths for various size and configuration requirements. So, whether an 8-slot or 14-slot backplane is needed, it can be implemented with little additional effort. With modular plastic card guides, the slots can be set at various pitches (distance between each slot), so configuring the spacing to accept compact, mid-size and full-size modules is simple. The plastic card guides provide the required electrical insulation and enlarged openings, resulting in a better cooling performance than many stamped chassis.

Modular MicroTCA also allows single and double modules to be used in the same chassis. Two single modules can be stacked on top of each other, thus occupying the same space as a double module in that slot, while double modules occupy other slots. To achieve this, there needs to be a set of divider plates to take on the middle strut and middle card guide, providing the top support for the lower single module and the bottom support for the upper single module (similar to an ATCA carrier board). The divider plates are mounted to the extrusions via the card guides. This is attractive for applications where mixing and matching the single and double modules is needed, or in development systems (see Figures 2a and 2b). Another design possibility with modular MicroTCA is stacking two sets of single modules on top of one another in every slot. With a modular design, extrusions can be placed at the top of the lower AdvancedMC module and at the bottom of the upper AdvancedMC module. This allows for a nice and very economic solution without having to use the divider plates. As with any of the modular designs, if an application ramps up to high volumes, the chassis designer can always switch to sheet metal for economies of scale.

Flexibility in Backplane Choices

Flexibility continues through to the backplane when using modular MicroTCA chassis components. First, the backplane can use three different types of connectors: pressfit, surface mount, and compression. Different customers and applications may demand different connector styles. The compression type connector is an attractive solution because of its flexibility. This connector is screwed-down to the backplane and connects via compression to the backplane. If the connector is damaged or broken, it can easily be unscrewed and replaced with a new one. The MicroTCA backplane also can be optimized for different data rates. For example, a backplane may be designed to handle 3.125 GB/s speeds and perform at that level. On the other hand, the backplane could be optimized for 6.250 GB/s speeds. In the latter case, the backplane may use a high-grade laminate material with a low dielectric value. This helps maintain cleaner signals when analyzed using signal integrity tools. Or, the backplane may have backdrilling or vias to reduce the effect of the stubs. The longer the stub length, the tougher it is to maintain clean signals. Any of these methods for improved performance also increase the costs. So, it makes sense to develop the backplane according to the needs of the customer’s specification.

In short, modularity through extrusions, plastic card guides, divider plates, connectors, and backplane materials is a way to manage the MicroTCA design process while controlling development costs associated with enclosures, supports, chassis, and backplanes. A modular design can save time, effort, and money when customizing a MicroTCA solution. Instead of starting from scratch, the designer is building from a proven platform and only making minor modifications. Modularity can also provide a wider range of options, like mixed single and double modules in the same chassis. It also can mean flexibility in backplane and enclosure configurations.

This article was written by Walter Schindler, Mechanical Engineering Manager, at Elma Electronic in Fremont, CA. For more information, contact Mr. Schindler at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit http://info.hotims.com/10968-400.

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