VME VPX And Beyond

3U VPX: Small, Rugged and REDI

With so many new architectures and form-factors there often seem to be a multitude of solutions for every application. One new form-factor may not be getting the attention it deserves because it has been in the shadow of its larger 6U sibling. It is 3U VPX.


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3U VPX is much more than just a half-size version of 6U VPX. Unlike 3U VME’s relationship to 6U VME, 3U VPX can support the same speed and bandwidth of its larger 6U brother. Unlike the trade-off between 3U cPCI and 6U cPCI, 3U VPX doesn’t require the user to make such a dramatic decision between bus width and I/O capability.

It is true that 3U VPX has a much more limited number of potential I/O channels than the larger 6U version. However, with two fabric connectors offering 32 differential bi-directional channels (64 links total), 3U VPX probably has more than enough connectivity for most applications.

One popular system configuration that a number of individuals have inquired about is a 3U VPX system with four x4 transport channels for the card to card topology and the remaining 16 bi-directional channels used for rear panel I/O.

Mechanical Form-Factor

When the features of 3U VPX are examined, the strength of this compact architecture becomes clear. The mechanics of the form-factor are particularly compelling when compared to other small serial fabric solutions (Figure 1). 3U VPX compares favorably to the other smaller form-factors with regards to total available PCB area as well as total front panel space as shown in Table 1.

The 3U VPX module is based on Eurcard packaging standards defined by IEEE 1101.1, IEEE 1101.10 and IEEE 1101.11. This is a widely supported modular packaging system that is also used by VITA 1.0, 1.1, 41.x, 46.x, CompactPCI, CompactPCI Express, PXI, PXI Express, VXI and VME in Physics. The modular approach is cost-effective for prototyping and small volume production but has also been executed cost-effectively in sheet metal for higher volume requirements.

One ergonomic success of the 3U x 1.0” Eurocard front panel is a very rugged insertion/ejection lever handle with an integrated micro-switch. Front panels with bezel opening to allow access to a PMC mezzanine card are also readily accomplished with Eurocard front panels as narrow as 0.8”.

While many other form-factors are still grappling with how to best accomplish conduction-cooling, 3U VPX conduction-cooled modules are already available from two suppliers. VPX modules can be built to the older IEEE 1101.2 conduction-cooling standard or the newer VITA 48.2 or 48.3 configurations. VITA 48.3 even provides a design for liquid flow-through cooled 3U VPX modules.

Another feature that is attractive for 3U VPX is that the form-factor is compatible with existing PMC mezzanine modules as well as the newer XMC mezzanine modules. The substantial existing ecosystem of PMC modules means that many interface standards can be supported immediately, through the use of either XMC/PMC carrier cards or VPX SBC cards with a PMC/XMC socket.

The MultiGig Backplane Connector

The pin and socket backplane connector used by VPX has been on the market for a number of years and is also being utilized by the VXS architecture. This modular connector is available in versions for differential signaling, single-ended signaling (many system management signals are single-ended) as well as for high power. The connector has recently been tested by a number of end users and has met all requirements for shock, vibration and signal integrity. Because it is a press fit connector with a footprint designed for the demands of differential signaling, it is efficient to install, and backplanes can be repaired if necessary. The design of the MultiGig connector system was specifically focused at avoiding damage during insertion as well as for superior electrical performance.

The flexibility of the backplane interconnect topologies is an important feature. The 3U VPX architecture has three backplane connectors in each slot. The J0 contains power at three voltage levels and such system signals as geographic address, power fail, JTAG and a reference clock. The other two connectors provide a total of 64 differential signal pairs and associated reference grounds (Table 2).

The J1 and J2 connectors can be configured as x1, x2 or x4 channels. The MultiGig connector supports XAUI 2.5 Gbit/s signaling and is scaleable to at least 6.25 Gbit/s. Because the VPX architecture allows the end user to define how the differential channels in J1 and J2 can be used, there is great flexibility to emphasize either channel interconnect mapping or rear panel I/O.

The layout of both the J1 and J2 connectors is shown by the chart of pin assignments in Table 2. It should be mentioned that the J1 and J2 connectors can also be configured entirely for single-ended applications. However, even when configured for differential signaling some single-ended pins are always available.

Channel Interconnect Topologies

The most common configuration for 3U VPX cards that has been requested and described in the literature thus far is for two x4 10 Gigabit channels at the top of the J1 connector. A single 10 Gbit/s channel is far more bandwidth than is available in previous parallel buses and is even more bandwidth than a PCIe graphics card would require. The remaining channels in J1 together with those in the J2 leave 24 XAUI-capable bi-directional ports that can be utilized entirely for rear panel I/O if desired.

Traditionally, a channel comprised of one or two bi-directional links is referred to as a “thin pipe.” A channel comprised of four bi-directional links is referred to as a “fat pipe.” The 24 thin pipe channels remaining can be used by a PMC or XMC mezzanine or simply allow the 3U VPX modules to interface with external equipment, networks, displays or storage systems. For more complex topologies, the entire J1 connector can be configured as four 10 Gigabit channels for slot-to-slot transport. This still leaves the 16 XAUI-capable bi-directional thin pipes in J2 for user I/O.

The number of possible topologies for the VPX architecture is only limited by the imagination of the system architect. A few examples shown here are twisted rings (Figure 2a), meshes with a star fabric overlay (Figure 2b), and dual rings (Figure 2c).

Figure 2d shows a hybrid topology that combines a fat pipe fabric with a star or dual-star thin pipe fabric. This particular topology is being standardized in the VITA 46.20 committee. Vita 46.20 adds a secondary star or dual-star thin pipe network. This secondary fabric is used for out-of-band signaling via Gigabit Ethernet. Such a secondary fabric can be used for system management functions and for other purposes such as software and firmware upgrades to individual cards. This is similar to the base fabric that has been a key feature of the AdvancedTCA architecture from the beginning.

The topology illustrated in Figure 2b could represent the VITA 46.20 architecture. Notice that an additional slot of CompactPCI Express is shown in a slot following slot 6. There is no reason why the thin pipe star defined in VITA 46.20 could not be used for PCI Express rather than Gigabit Ethernet. Making the thin pipe network PCI Express would make it particularly convenient for bridging 3U CompactPCI Express to a 3U VPX system, thereby increasing the number of available boards for configuring complete systems. In fact, many of the 3U VPX boards announced thus far utilize cPCIe signaling, making this hybrid cPCIe concept a very realistic possibility.

Power Capability and Flexibility

Connector J0 provides 3.3, 5.0 and 12-volt power as well as utility signals (Table 3). It is allowed to bring as much as 275 watts onto a module, which would require an advanced cooling approach such as VITA 48.3. However, this upper limit is only due to cooling; the connector design and assignments could supply even greater power to a module, if there were a way to cool such a module.

There are two factors that should always be taken into consideration when making an architectural decision. The first is the available ecosystem of compatible components. A program manager must evaluate the level of support that is available or expected to be available by the time the system under consideration must be deployed.

The second consideration is the specific identity of the suppliers and partners who will be expected to provide system components. Ideally, when a program manager conducts an inventory of VPX suppliers, he or she will find that their existing favored suppliers are among the supporters of the architecture under consideration. After all, to become a favored supplier is the result of a complex layering of relationships, capabilities and operating practices over time. Your prime suppliers probably know you and your market areas already and are therefore likely to best understand your requirements for a new system under development.

VPX has a number of characteristics and features that will be of significant utility for the Mil-Aero and rugged industrial market segments. At the present time, there is no other small form-factor architecture with such a compelling collection of features. Making the architecture even more attractive is the fact that such cards are already on the market today and the schemes for conduction-cooling and rugged packaging are already defined in detail. A look at the companies already supporting this new architecture makes it clear that it will be a significant contender for Mil-Aero and industrial rugged applications requiring high-speed signal processing, and a compact, rugged, front-accessible card size.

Elma Bustronic
Fremont, CA.
(510) 490-7388.