VPX6 (or 6U VPX) Has Its Place in SWaP-Constrained Applications
The 3U form factor is commonly touted as the format of choice for high bandwidth applications that must work within the constraints of SWaP (size, weight and power) considerations; however, in some such cases the 6U form factor of VPX may be the answer and actually take up less space.
VPX is the ANSI/VITA 46 standard that incorporates high-speed serial fabrics to facilitate high volumes of data transmission. Like its predecessor, VME, it is available in two sizes, 3U (VPX3) and 6U (VPX6). It offers designers the license to choose the format best suited to each application.
Although it appears that 3U VPX offers about half the board space of 6U for plug-ins, in reality the board space is significantly less. The outline of a 3U printed circuit card is 100 mm x 160 mm, compared to 233 mm x 160 mm for a 6U card. However, designers must take more into consideration when determining the actual amount of printed circuit board real estate available for data transmission. For instance, they consider the amount of functional surface area that becomes unavailable to them when it is taken up by ancillary components that have no direct data role. Examples of this include the wedgelocks that play a role in conduction cooling, which is a necessity for data-intensive, high bandwidth defense and aerospace applications. These wedgelocks cover the vertical edges of boards.
Other factors that remove significant board space from play are the depth of backplane connectors, mezzanine boards, onboard voltage converters and large devices, such as microprocessors, FPGAs and host bridges. These large devices in particular pose a problem with the smaller cards, as opposed to the roomier VPX6 cards.
When these kinds of factors are accounted for in both the 3U and VPX 6U boards, the space available for data applications on a 3U card can end up shrinking from half the space of the 6U card to about a third of the space. With this in mind, an embedded computing system that requires three 6U boards could require up to nine 3U form factor boards. In effect, the 3U alternative would end up requiring 150 percent of the board space of the 6U solution.
For such a situation, 6U VPX (or VPX6) boards may be the preferred SWaP solution, taking up less space than the smaller 3U format.
Nevertheless, the 3U VPX boards have its place in small VPX systems. For instance: systems that require six or fewer boards; conduction-cooled systems, as smaller boards provide a shorter route for heat to travel out; and applications subject to shock and vibration.