To successfully implement a distributed system, you need to consider its measurement capabilities, the characteristics of the selected bus and its software framework.

Measurement capability

When selecting an instrument, the primary consideration is to make sure it meets your requirements for accuracy, sampling rate, bandwidth, dynamic range or other measurement requirements. Stand-alone instruments typically provide a fixed set of functions the vendor has defined for the system. However, a modular instrumentation approach takes advantage of software and generic PC hardware to provide the computing power for calculations and analysis on the raw data from the modular hardware. 

Bus characteristics

When evaluating bus technologies for an application, engineers must consider key attributes such as latency, bandwidth, software support, distance support and availability. Latency and bandwidth, in particular, have a large impact on performance. Latency measures the delay of transmission of data across a bus, while bandwidth measures the rate at which data is sent across the bus, typically measured in Mbyte/s.

Latency has a direct impact on applications such as digital multimeter (DMM) measurements, switching and instrument configuration. Bandwidth is important in applications such as waveform generation and acquisition, as well as RF measurements, in order that the raw measurement data can be communicated to the PC memory for processing and analysis.

GPIB has a well-established position as a bus to be used in a test system, however in modern computing systems there are many more options that offer additional advantages. Ethernet or Lan are also mature buses and for many years have been used in test and measurement applications to distribute traditional boxed, PC-based or PXI-based instruments. Typically, these systems are used to measure large devices (for example, airplane wings) or geographically distributed systems (for example, regional power grids or cyclotrons).

In 2005, a new Ethernet-based standard, Lan Extensions for Instrumentation (LXI), was introduced with the goal of increasing interoperability. It uses many other standards including VXI-11, IEEE 1588 and HTTP. USB has become a popular communication bus choice for stand-alone instruments because its widespread availability on PCs and plug-and-play ease of use allow engineers to quickly connect and configure USB-based instruments. In addition to easy connectivity, you can achieve a maximum transfer rate of 480Mbit/s with USB 2.0.

The PCI bus offers a number of advantages over the previously discussed buses, including low latency and high bandwidth at 132Mbyte/s.

PXI, PCI eXtensions for Instrumentation, adds a 10MHz reference clock, an 8-line trigger bus and star trigger lines with intermodule skew of less than 1ns.

An evolution of PCI, PCI Express delivers data transfer rates of up to 4Gbit/s; dedicated, bi-directional bandwidth per slot and complete software compatibility and PXI Express integrates PCI Express into the backplane.

Flexible software framework

When combining instruments across a range of buses, it is important to use a flexible software framework that enables easy integration. A framework using the LabView graphical development environment and NI TestStand test management software, communicating to instruments via an integrated measurement and control services layer, enables engineers to develop distributed test systems consisting of PXI-based and traditional fixed-functionality instruments.

Tristan Jones is technical marketing engineer at National Instruments UK & Ireland