VPX: The Rugged Bus Architecture for Embedded Military Applications

For over two decades, the VMEbus has maintained its position as the open standard bus architecture of choice for defense and aerospace applications. While VME, a parallel bus, promises to continue its reign as the most popular embedded board standard for years to come, it has become clear over the last several years that, for an emerging class of very demanding highend applications such as radar and signal processing, a new higher-bandwidth bus architecture is required.

In response, Curtiss-Wright, prime military integrators, and other COTS industry leaders jointly developed the VPX (VITA 46) standard and its complement, VPX-REDI (VITA 48). VPX combines VME's successful 6U form-factor (a 3U variant also is defined) and 0.8" pitch chassis ecosystem with a new highbandwidth backplane connector, capable of supporting high-speed interconnects and serial switched fabrics such as Serial RapidIO (SRIO) and PCI Express (PCIe) to address the most demanding single-board computer (SBC) and DSP engine applications.

Ruggedized for the Field

Figure 1. Example of a 3U VPX/VPX-REDI single-board computer.

Compared to VME, VPX also provides a significant increase in user-definable I/O pins. These new standards were created to specifically meet the intense ruggedization requirements of the most demanding military applications. Unlike the traditional VME DIN-style connector, VPX and VPX-REDI use a new connector type that provides built-in electrostatic- discharge (ESD) protection of sensitive electronics. Adding a further level of ruggedization, VPX-REDI is the first open board standard to define the use of top and bottom metal covers to mitigate electrical and physical damage during in-the-field repair and replacement for handling protection and fault isolation.

The key to VPX's performance is its use of Tyco Electronics' 7-row MultiGig RT2 "wafer-style" connector. The MultiGig RT2 provides the increased I/O pins and bandwidth required to handle high-speed signals and serial switch fabrics such as SRIO, which can require performance up to 3.125 Gb/s. This connector features tightly controlled impedance, low insertion loss, and less than 3% crosstalk at signaling rates up to 6.25 Gbaud.

The MultiGig RT2 connector's ESD ground plane and contact layout are designed to protect signal contacts and prevent accidental discharge during handling. The connector features a thin ground strip located on the leading edge of the signal wafer on the opposite side from the signal contacts. To ensure that it can perform optimally in hostile environments, the connector was subjected to a rigorous series of environmental tests conducted by a third party, Contech Research. The tests included analyses of the connector under the following conditions:

  • Vibration (power spectral density of 0.1 g2/Hz from 50 Hz to 2 kHz, 1.5 hours/axis)
  • Shock (50 g, 11 ms)
  • Temperature (-40°C to 100°C)
  • Humidity
  • Dust and sand
  • Electrostatic discharge (ESD) protection

These tests showed that the insertion force of a 6U VPX board is only about 18% greater than for a VME64x card, and about the same as that for a VXS board. The connector also features alignment pins that virtually eliminate the possibility of a broken or bent connector pin sometimes experienced with VME64x or VXS. The test results are contained in a connector and module qualification report available from the VMEbus International Trade Association (VITA) and its Standards Organization (VSO) and can be downloaded at the VITA Web site at www.vita.com/vpx.html.

Figure 2. Tyco’s MultiGig RT2 connector’s ESD ground plane and contact layout are designed to protect signal contacts and prevent accidental discharge during handling.

The optional top and bottom covers defined by VPX-REDI provide additional ESD protection as well as physically protecting the board components. Use of these covers enables VPX-REDI boards to support in-the-field 2-Level maintenance (2LM), or card-level replacement, which is highly desired by the military for the significant time and cost savings it offers for maintenance of electronic subsystems. The more costly and complicated alternative, typically used today, is to completely remove an entire module-based line-replaceableunit (LRU) subsystem. The faulty subsystem is then serviced remotely to avoid mishandling or damage from static electricity. This approach requires having costly LRU-level spares available, as opposed to the simpler method of replacing individual cards, or linereplaceable modules (LRMs) in the field. VPX-REDI enables COTS LRMs to be replaced directly and safely in previously prohibitive environments such as the static-rich deck of an aircraft carrier, and requires no special training or tools.

Benefits of an Open Standard

VPX also provides a significantly greater number of user I/O pins than are available from the VME64x or VXS (VITA 41) standards. A 6U VPX board provides a total of 464 signal contacts and more than 200 ground contacts. It also supports up to 192 high-speed differential pairs. For 6U VPX, the connector offers 328 independent connections to the backplane, plus the required ground connections. Of these, 320 (160 pairs) are routed to support differential signals. In comparison, the legacy VME64x standard provides no differentially routed connections to the backplane.

In addition to high-speed signals for serial switch fabrics and high-speed I/O, VPX is also able to support legacy VMEbus or PCI bus protocols for commercial and military systems. Support for these legacy parallel buses allows VPX users to upgrade their existing VME systems using hybrid architectures that protect their investments in COTS VMEbus and PCI-based modules.

VPX and VPX-REDI uniquely deliver 2LM-level ESD and handling protection combined with the through-life cost benefits of open standards and a competitive supplier base demonstrated over the years by the success of the VMEbus community. The new VPX and VPXREDI standards, supported by the major military systems integrators and COTS suppliers, will create the broad infrastructure needed for programs to adopt LRMs as their first choice, even for small volumes, with the proven confidence of future-proofed technology and cost structures offered by the COTS industry.

This article was written by John Wranovics of Curtiss-Wright Controls Embedded Computing, Charlotte, NC. For more information, Click Here