Exploring VXI 4.0

The popularity of the venerable VXIbus in large-scale ATE and data acquisition applications traditionally has been rooted in its stability that spans a 23-year history of product introductions. With the release of the VXI 4.0 specification, the VXI Consortium continues to offer customers a stable instrumentation platform that preserves backward compatibility. It also delivers exceptional performance enhancements that surpass comparable modular architectures for the application spaces it is intended to address.

The flexibility offered between a host controller and a mainframe is a clear differentiator that the VXIbus maintains over other modular instrumentation platforms. Over the years, system developers have communicated to VXI mainframes over GPIB, MXI/MXI2, and a variety of consumer-oriented buses such as FireWire, USB, and Ethernet. With a well-defined plug&play software architecture, end users have been able to upgrade their systems to the latest bus technologies with little or no impact to application code by simply swapping Slot 0 interfaces.

The evolution of operating systems also has required vendors to keep current with their software drivers to ensure that end users benefit from the features of the new OS’s capabilities. National Instruments recently released Windows 7 64-bit support for its MXI Express Slot 0 interface, and VTI Instruments did the same for its LXI/VXI bridge. By adding 64-bit support to VXI, memory-intensive application programs can use the additional RAM to improve overall system performance.

VXI 4.0 added significant throughput capability with the adoption of the 2eSST protocol and the subsequent 320-MB/s transfer rate it enables. The 2eSST protocol also accommodates broadcast data transfers, allowing the master to transfer the same data to multiple slaves with a single transfer instead of repeating the transfer to each slave.

Bustec’s PCI-Express/VXI bridge uses this new throughput capability to satisfy large channel count, high-speed data acquisition application requirements. This Slot 0 device, also with Windows 7 64-bit support, connects to the host controller via a four-lane, low-latency (400 ns) PCI Express serial communications link, providing yet another option for remote control of instruments.

The VXIbus incorporates well-defined power and cooling specifications that make VXI the platform of choice for performance-driven applications. As more demanding applications emerged, a primary focus of the VXI Technical Committee was to increase the power capacity on the backplane.

To do this, the 96-pin J1/J2 connectors were expanded to 160 pins. Two outer rows were added to these connectors to achieve complete backward compatibility with legacy VXI instruments. The additional pins increased the traditional power rail capacities by 418 W while adding 3.3 V and user logic rails. This has proven particularly useful in multichannel high-speed digital applications that operate at various voltage and current levels in which each channel can be dissipating up to 5 W.

With the additional power that VXI 4.0 offers, EADS Test and Services has introduced a 50-MHz digital test instrument that avoids limitations or trade-offs that exist with digital test products on other platforms. All power required to source 80-mA line regulators and high slew rates is derived directly from the VXI backplane; no external power source is required.

The longevity of the VXIbus leads to the natural assumption that it is based on technology that is long in the tooth. Instead, the specification has demonstrated the capability to adapt to emerging technology while maintaining compatibility with its extensive installed base. With the adoption of VXI 4.0, vendors are incorporating the benefits the new specification offers into their products and further establishing the VXIbus as a viable platform for demanding test and measurement applications.

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