RTC Magazine

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.