AdvancedTCA Extensions: Extending the ecosystem

4The ATCA Extensions group has been busy expanding the power and space allowances beyond the existing ATCA 3.0 specification, resulting in a double slot width form factor for the front and rear boards as well as mechanical changes to the subracks that offer a substantial component real estate increase over the ATCA 3.0 RTM form factor. Yet, with all of these advantages come legitimate concerns regarding the cooling of these larger and more powerful blades and the subrack as a whole.

The ATCA Extensions subcommittee has been hard at work expanding the AdvancedTCA architecture. Among these improvements, Extensions offers a wider, double-slot form factor, increased PCB real estate on the rear blades, increased maximum power (to as much as 800 watts per blade), and enhanced cooling management. As we take a look at some of these in more detail, we will examine some of the challenges and how they are being tackled.

Reform factor

Let’s start with the support of a double-wide blade, namely a single blade comprised of two standard 6HP slots. While this is something we have seen done numerous times in AdvancedTCA subracks, where one may need additional component, I/O space, or mezzanines, the Extensions specification standardizes this form factor as an option, outlining the front panel dimensions, datum locations, and tolerances, as well as the increased component area and available mezzanine envelopes for consistent implementation. This new form factor provides the option of locating the PCB in either the left (bottom) position of the front panel to maximize the available Component Side 1 height envelope, or centered to utilize an expanded component envelope for Component Side 2. In addition, this facilitates the ability to develop even wider, multi-slot configurations[1].

Additionally, a substantial increase of PCB real estate is available on the rear modules due to Extensions’ support of standard AdvancedTCA front boards. Boards can be implemented in the front and rear of the subrack, complete with Zone 1 power and Zone 2 data transport connectivity. This, in concert with other configurations, more than triples the component area available on the current AdvancedTCA Rear Transition Module (RTM). As a way to differentiate the two sides of the subrack, Extensions designates the front as “Side A,” and the rear (RTM in the AdvancedTCA specification) as "Side B" (Figure 1).

Figure1
Figure 1: The AdvancedTCA Extensions pitch envelope is offset from the front to rear of the subrack, contrary to the original AdvancedTCA spec.

In order to facilitate the use of front boards in this new Side B configuration, it was necessary to adjust the orientation of the pitch line from front to back. As we know, the existing AdvancedTCA spec maintains the same pitch envelope from the front boards to the RTMs. With the new Extensions configuration, the front side pitch for each slot stays exactly as it currently is with the AdvancedTCA spec, but the rear side is offset a little more than a millimeter. In short, the reason for this is to maintain an equal front-to-rear overall subrack aperture width and gasket interface aperture without requiring any additional subrack features or increases in overall subrack width.

Maintaining faceplate orientation of a front board in the rear of the subrack results in the PCB not being aligned in the same plane front-to-rear. This, along with the pitch offset, presents some challenges with Zone 3 interconnectivity. This may be addressed with some unique interposer designs, and the specification will be kind enough to actually provide some examples of how this can be done. Other solutions could be connector based, but this would require new connector designs, so the level of demand will dictate the availability of those solutions.

Extended opportunities

Anyone that has ever had to contend with the limited real estate offered by the AdvancedTCA RTM will be interested in the new specification for the “Extended Transition Module.” This blade was developed for use in the rear (or Side B) of the subrack. This is, in essence, a full front board configuration with an extension area for direct mating to the front board through Zone 3. Another new configuration is the “Extended Board,” a blade that supports Zone 1 Power and Zone 2 Data Transport, along with the Zone 3 interconnectivity to a front board[2].

Of course, these new rear board options will need the support of additional backplane configurations. Supporting the implementation of Extended Transition Modules, the subrack utilizes the same backplane as the current AdvancedTCA 3.0 specification for the front, but one with an increased depth in the rear. This configuration is categorized as a “Single-Sided Backplane Shelf.” There is also a “Dual Backplane Shelf” that uses a separate Side B backplane for connectivity to rear blades, and would support the use of standard front boards along with the new Extended Board. The option of a “Monolithic Backplane Shelf” – a single backplane supporting both front and rear blade interconnects – is also available. With this configuration, the thickness of the backplane would dictate the total shelf depth.

As you can imagine, all of these advancements vastly expand shelf configuration options. Along with these improvements comes an increase in power, from a previous maximum of 400 watts to a possible 800 watts per blade. This, coupled with the expanded shelf depth, could prove to be a challenge for thermal management.

Keeping it cool

Air cooling via forced convection will continue to be the most simple and cost effective method used, but maintaining consistent airflow distribution through all blade regions for Side A and Side B blades will no doubt require additional consideration.

One area of concern is in the vicinity of the backplane, due to the additional space added between the front and back of the subrack for supplemental backplane configurations. This additional space will have to be sealed off, thereby preventing air from escaping between the backplanes (the open area between Side A and Side B). Figure 2 displays this blockage and the subsequent airflow.

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Figure 2: An example of airflow through an ATCA Extensions subrack.

Most likely, this will be managed with features added to the subrack structure, and while the Extensions specification does not specify exact methods for accomplishing this, recommended locations for these features are outlined for guidance.

For extended blades with Zone 3 interconnections, restricting front-to-back airflow through this interface area will also be important. While the connectors could inherently block some of this airflow, additional air blocking features will most likely be required. While this is not all that different than the initial AdvancedTCA spec, the way this is handled may vary somewhat. AdvancedTCA defines an airflow seal for unused slots that is attached directly to the backplane support bar in the subrack, accessible from the rear. As Extensions offers a much greater depth in the rear section, getting to this area, especially if slots are populated, may prove very difficult. So what are some ways that airflow can be managed in this area?

A critical concern has always been the management of unused slots. While there are ways to restrict airflow within the chassis structure itself, primarily front panels with air restrictors (or “baffles”) are used. This allows various shelf configurations to be easily managed in the field. One of the problems noted with these devices has been the wide variety, and frankly inconsistency, of existing designs. To better control the consistency of these fillers, the Extensions Committee has worked on defining a “Universal Filler” displayed in Figure 3. This design is focused on providing maximum air blockage for unused slots in either the single- or double-wide applications, as well as setting a consistent location for the air impedance feature.

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Figure 3: The “Universal Filler” is aimed at becoming a consistent ATCA/AdvancedTCA Extensions air restriction tool.

This design of the Universal Filler is not exclusive to the Extensions chassis, and can be used in the front of any ATCA-compliant subrack. Also included is a feature to better control the location of this dummy blade in each slot by interfacing with the backplane alignment keys. To address the concerns with the difficulty of managing the Zone 3 airflow mentioned above, the Universal Filler also includes a feature to block front-to-back airflow in the Zone 3 area. This will serve to seal Zone 3 on both Side A and Side B of the subrack for both single- and double-slot form factors. All of the features built into the Universal Filler will result in forcing as much air as possible to active blades for maximum cooling of those slots.

Enhanced outlook

The ATCA Extensions specification will introduce some exciting enhancements: PCB and front panel space, blade configuration options, increased power ... and all of these improvements need provisions for viable component and subrack cooling solutions. This new specification provides that, and presents an easy-to-implement expanded platform for implementation in new applications.

Matthew Travers is an applications engineer at CBT Technology and has more than 20 years’ experience designing front panels and mechanical systems for the telecommunications industry.

CBT Technology

www.cbttechnology.com

MTravers@cbttechnology.com

 

References:

[1] Given the increased width of the front panel, it has been noted that some sort of simple bracket may need to be added to maintain structural rigidity and perpendicularity between the PCB and front panel.

 

[2] Both of these boards are required to include a feature protecting them from inadvertent insertion in a Side A slot opposing a previously installed Extended Transition Module, as this has the potential to cause damage to the blade connectors if they come in contact with an installed Side B Extended Board.