VXI gets a boost

In December 2003, the VXIbus Consortium (www.vxibus.org) announced specification VXI 3.0, which doubles the bus's data rate to 160 Mbytes/s. Although the 80-Mbytes/s rate of VXI 2.0 is adequate for most test systems, some existing applications will benefit from the higher speed, and applications that weren't feasible at the 80-Mbytes/s data rate will now become a reality.

The first instruments to attain 3.0 compliance will likely be high-speed, multichannel digitizers. Applications such as aircraft vibration testing, in which hundreds of analog channels must be digitized, will be among the first beneficiaries. Parallel digital data-bus analyzers could also potentially benefit from the increased data rate, says Chuck Reynolds, VXI program manager at Agilent Technologies (www.agilent.com).

VXI-based test systems will get a speed boost as VXI 3.0-compliant products enter the market. Courtesy of Racal Instruments.

To take advantage of VXI 3.0's higher throughput, you can use your existing chassis, but you'll need a slot-0 controller or bus master and I/O modules that support the VXI 3.0 protocol. In systems that use a VXI 3.0-compliant bus master in addition to a slot-0 controller, you can transfer data between VXI 3.0 master and slave modules at 160 Mbytes/s. You can transfer data between VXI 3.0 I/O modules and a VXI 2.0 slot-0 controller module at 80 Mbytes/s.

When VXI 3.0-compliant I/O cards and slot-0 controllers become available later this year, you'll get the best of both worlds—better throughput with new cards and backward compatibility with existing cards. That's because systems with VXI 3.0-compliant modules will communicate over the bus at an I/O module's maximum speed.

Also, you won't need to rewrite existing application software if you upgrade your slot-0 controller to VXI 3.0, because you'll get a new implementation of VISA that supports VXI 3.0 and is compatible with VXI 2.0-compliant products. Of course, you will need instrument drivers for the high-speed I/O modules.

In addition to becoming faster, VXI will get expanded addressing for system memory as the addressing scheme moves from VME32 to VME64. Your I/O card's bus logical address will remain at a maximum of 254, though, which is still more than a test system needs. The expanded addressing affects the system controller's ability to access VXI bus memory. The Consortium put this upgrade into VXI 3.0 to allow for future system controllers that could run 64-bit processors and enable future test systems to break the 4-Gbyte boundary, but this change generally won't matter in test applications.

Martin Rowe, Senior Technical Editor, m.rowe@tmworld.com