POTENT- BUT PRICE

When the IEEE adopted VXI in 1993, it faced two obstacles to acceptance. VXI was perceived as difficult to use and also expensive

You’d have expected the VXI virtual instrument architecture to lower the cost of instrumentation compared to traditional rack-and-stack systems. It eliminates redundant components like duplicate power supplies, displays, and embedded data processing. VXI maximises computer control while minimising hardware. But until recently VXI has remained a high-cost platform. Why?

VXI vendors achieved many of the goals they set out to accomplish way back in 1987. They delivered more compact, powerful and open systems – and VXI use did grow. However, many thought that VXI’s technical superiority alone would push it into the main-stream of instrumentation. It didn’t.

High costs and ease-of-use issues were hindering acceptance. So in 1993 the VXI community established the VXIplug&play Systems Alliance. Vendors focused on making VXI easier to use. The alliance successfully built industry-wide standards for software that made system development easier and software reusable on what were already powerful instruments. But it didn’t come free; indeed, it added more cost!

So VXI was effectively positioned as a high-end solution for test – to justify the higher costs. To reposition it as a mainstream platform, vendors had to deliver products that were competitive. National Instruments’ approach was to introduce the VXI-DAQ instrument modules, harnessing the volume sales of PC-based DAQ products and combining this with its VXI controller technology. The result was VXI products that were higher performance, but lower cost.

The PC revolution had already affected VXI from the controller perspective – the most popular and powerful VXI controllers were PC-based. They were less expensive and more powerful than proprietary architectures. Similarly, look at application software development packages. Those for the PC, such as LabVIEW, LabWindows/CVI, Visual Basic and Visual C++, are inexpensive.

But, PC-based DAQ boards have themselves increased in performance and capability. For example, National’s instrumentation-class E-Series boards compete with – and in some cases surpass – GPIB-controlled instruments. And, the PC DAQ instruments are general-purpose, so you can customise. You can program an A/D PC DAQ board as a `scope, voltmeter, or digitiser. And, because of the tight coupling between the board and the computer, the software controls the PC DAQ board as if it were embedded.

Integrating PC DAQ technology into VXI instruments, National now combines PC DAQ technology from the E-Series with the high-performance VXI controller interface from the MXI-2 and VXIpc 850 embedded VXI controllers. In particular, the VXI-MIO Series integrates the DAQ-STC (a counter/timer ASIC) and the NI-PGIA, an E-Series fast-settling programmable gain amplifier, to achieve superior analogue performance.

With general-purpose hardware, PC DAQ boards require a general-purpose instrument driver so that users can control the boards without having to write register-level code. An example is the NI-DAQ software which makes PC DAQ boards easy to use via an application programming interface (API) that is designed to exploit board-level functionality.

This provides an intuitive software interface so you can perform analogue I/O, digital I/O and counter/timer operations from a number of application development environments. In many cases, waveform acquisition or waveform generation can occur in as little as one function call or virtual instrument.

The cost of a VXI system includes not only hardware and software development – but also the learning curve. The step from GPIB to VXI may appear difficult. But the VXI Consortium even defined special extensions to the specification, such as the Word Serial Protocol, to make this transition easier and less costly.

But now that a common software architecture exists between PC DAQ and VXI instrumentation, the transition to VXI has been further eased. You can start developing systems on the PC, then move to VXI as the needs of the system expand – the source code moves directly.