Understanding Modular Instrumentation and Traditional Instrumentation Architectures for Automated Test Systems

Scientists and engineers everywhere have a need to measure, record, and analyze the world around them. Their environments range from analyzing the analog and digital signals of a circuit to measuring the vibration of an airplane’s engine. Over the past several decades, the approach to solving these applications has evolved from the use of primitive instrumentation, such as analog meters and simple transducers, to the sophisticated architecture of today’s instruments. Almost all modern instruments used for automated test systems can be categorized as either a modular instrument or a traditional instrument. This paper examines the four key components found in both traditional and modern instrumentation and discusses the benefits of these different approaches.

Modular and Traditional Approaches to Instrumentation

Fundamentally instrument vendors use one of two approaches in developing measurement hardware: modular instrumentation or traditional instrumentation. Traditional instruments, also referred to as standalone instruments, integrate all of the components into a single device, such as the user interface, power supply, CPU, and measurement hardware. These instruments are typically designed for quick measurements in a laboratory or deign workbench environment.

An automated test system typically integrates multiple instruments together controller by a central PC. Figure 1 illustrates what a typical test and measurement system looks with just two traditional instruments connected through a standard peripheral bus. Notice that many of the common components, such as the interface, power supply, and CPU, are found in both traditional instruments. This means that each instrument replicates the cost of these common components and must be built large enough to accommodate all of pieces.

By contrast, Figure 2 shows the modular instrumentation approach that relies on PC technology to provide the common components of the instruments. A modular instrument only contains the measurement hardware needed to acquire or generate the raw data. They are peripheral devices that connect to a central PC through standard interfaces such as the PCI bus or a USB port in order to build an automated test system. Adding additional measurements simply require adding another peripheral device to the system as oppose to buying a new system that includes a new display, processor, and other common components, as is required for traditional instruments. This fact enables modular instrumentation to better address the needs of large, complex automated test systems.

The term virtual instrumentation refers to an automated test or measurement system that uses software on a PC-based host system to control the hardware, such as the modular instrumentation in an automated test system. In a virtual instrument, the hardware control, data analysis, and data presentation are handled entirely by user-defined applications. These applications can be written in standard application development environment, such as NI LabVIEW, C/C++, or .NET, on the host system. Data is passed from the instrument to the host system through standard I/O buses, like PCI or PCI Express.

In general, the modular and traditional instrumentation approaches have four main components that must be understand when comparing one instrument to another: measurement hardware, user interface, software, and connectivity. This paper examines how these four parts are architected into a traditional and modular instrumentation.

Components of an Instrument

Common Component	Description
Measurement Hardware	Performs the generation or acquisition of raw signals. Typical measurement hardware includes analog-to-digital or digital-to-analog converters, filters, and attenuators.
User Interface	Provides the means by which someone controls the instrument and views the current state of the measurements and analysis. The UI can be fixed controls on a traditional instrument or a user-defined virtual instrument displayed on a monitor.
Software	Provides a layer of abstraction to simplify the analysis of a raw bit stream from hardware and the configuration of an instrument’s registers. The software can either reside in the firmware of an instrument or be a flexible, user-defined approach.
Connectivity	The means by which multiple instruments are connected together and to a host computer. Traditional instruments rely on peripheral buses, such as GPIB, USB, and LAN, while modular instruments typically use the PC I/O bus, such as PCI or PCI Express.

Measurements Hardware

The measurement hardware of an instrument is responsible for performing the generation or acquisition of a specific signal. For example, the most common instrument in use today is an oscilloscope, whose measurement hardware includes an analog front end to receive, filter, and attenuate a signal and an analog-to-digital converter (ADC) to covert the signal into bits. These raw bits are then interpreted and analyzed by a processor.

Both modular and traditional oscilloscopes contain the same type of measurement hardware. This hardware dictates the key properties of an instrument, such as what measurement is performed and how accurate those measurements are. While both instrumentation approaches may include the same measurement hardware, relying on a PC-based host system to provide the other common components reduces the cost and complexity of these instruments.

As shown in Figure 1, a traditional instrument’s architecture tightly integrates the measurement circuitry with the other components. The integrated components result in a fixed design that is often difficult, if not impossible, to update or repair. It is not possible to replace the measurement hardware of a traditional instrument if a new measurement is needed or higher performance is required. Engineers are forced to purchase a new instrument even if the existing processor, software, memory, connectivity, and interface are adequate for the new measurement.

Other than basic I/O hardware to interface with a bus, the measurement hardware makes up the majority of a modular instrument. By relying on commercial technology rather than the development of custom components, modular instrumentation vendors also achieve shorter development time for new instruments. Engineers for modular instruments only have to focus on the design and functionality of the measurement hardware; they do not have to worry about integrating in a user interface, power supply, custom bus, or other common components found on traditional instruments.

When comparing the two approaches to instrumentation, it is important to understand that while the implementations of the remaining three components of an instrument vary widely between the two approaches, the measurement hardware found on both instruments often provides similar functionality. The main difference is that traditional instruments dictate how a user can use the measurement hardware found on an device while a modular instrumentation approaches relies on software and user-defined applications to control how the raw data from the measurement hardware is analyzed and used.

User Interface

The user interface of an instrument provides the means by which someone controls the instrument and views the current state of the measurement and analysis. For example, the controls on an acquisition device allow a user to adjust the sampling rate, specify the number of channels, and to control the analysis performed on the acquired data. How the user interacts with the interface is very different between the traditional and modular instrumentation approaches. Figure 5 shows examples of the two interfaces: a traditional instrument user interface and a software-defined, virtual instrument interface.

The user of a traditional standalone instrument, sometimes referred to as a human-machine interface (HMI), requires a user to interact with physical controls, such as knobs and buttons, and indicators, such as LEDs and small displays, which are integrated into the instrument. The user interface is fixed and cannot be customized on traditional instruments. Traditional instrument vendors must try to design the interface so that users, from graduate students to operators in a manufacturing environment, can all easily use the instrument. Some modern standalone instruments are similar to a computer in that they are loaded with a standard operating system and can be controlled with a keyboard and mouse.

Because a modular instrument only contains the measurement hardware, the host computer provides the interface, often referred to as a graphical user interface (GUI). As shown in Figure 2, the modular instrument communicates with the host system through high-speed buses, such as a PCI bus or USB. The host system treats this instrument like a peripheral device, such as a sound card or Ethernet card, which means standard software can control, configure, and analyze data from the instrument.

The user of a modular instrument can create one or multiple user interfaces using standard application development environments, such as NI LabVIEW and C/C++. These development environments include virtual knobs, buttons, and indicators that can be arranged to mimic the look and feel of a traditional instrument or be designed to best meet the needs of the end user. One application can have a simple interface for a technician while another application can leverage the same measurement hardware and test system but provide detailed debug information and specific controls for an engineer. A keyboard, mouse, or touch screen are used to configure the instrument while a standard monitor displays the current state of the instrument on indicators, which can include anything from text boxes to 3-D graphs.

Software

The role of software in instrumentation is to provide a layer of abstraction to the user that makes it easier to analyze the raw bit stream from hardware and to store instrument specific settings into the device’s registers. Along with configuring the hardware, software plays a critical role in defining what measurements the device is capable of performing and how the results of those measurements are presented. The main difference between the software on a traditional instrument and modular instrument is where the software is located and whether it is vendor-defined or user-defined.

Traditional instruments provide predefined software that is usually embedded in the firmware of the instrument. Historically, very few traditional instruments allowed the user to manipulate or change this firmware, which meant that all of the capabilities of the instrument are fixed and predefined by the vendor. If additional analysis capabilities were necessary, then the user often had to purchase new software or rely on the vendor to develop additional functions.

A modular instrument system relies on user-defined software to control the hardware. User-defined software refers to custom applications that can be created in standard development environments, such as NI LabVIEW or C/C++. The software of a virtual instrument usually consists of multiple layers as illustrated in Figure 6. The software dictates the measurements performed in the automated system by converting the raw bit stream of data into meaningful information, such as an FFT or histogram plot.
Directly above the device I/O and computing layers, which includes the modular instrumentation and host system, is the measurement and control services software. This middle layer includes the I/O driver software, instrument drivers, and configuration software, which provides the connectivity between the development software and the actual hardware.

Directly above this software sites the application development environment, which is critical for rapidly deploying a test system. In particular, NI LabVIEW and its graphical programming approach have made it easier for engineers to decrease development time. Some applications also require an additional layer for system management. This layer is represented by the top of Figure 6 and includes management software like NI TestStand.

Connectivity between Instruments

The final common component found in both instrumentation approaches is the connectivity between instruments. An I/O bus is necessary to share data between instruments as well as with any host PC. A connection between instruments is also needed to share triggers and timing and synchronization signals. Most test systems today have numerous measurement needs, which make it necessary to incorporate many different instruments into a signal system. The connectivity between these instruments affects the test system’s accuracy, complexity, and development time.

When evaluating the I/O bus used by an instrument to transfer data, bandwidth and latency are two of the most important bus characteristics. Bandwidth measures the rate at which data is sent across the bus, typically in MBytes/s, while latency measures the inherent delay in data transmission across the bus. These two characteristics affect whether data can be sent as fast as it is acquired and how much onboard memory an instrument needs. Latency, while less observable, has a direct impact on applications such as digital multimeter (DMM) measurements, switching, and instrument configuration, since it affects how quickly a command sent from one node on the bus, such as the PC controller, arrives at and is processed at another node, such as the instrument.

In an automated test system, traditional instruments require a peripheral bus to connect them to a host computer. The general purpose interface bus (GPIB) has historically been the standard for instrument connectivity; however, newer buses, such as LAN and USB, are being used in some newer standalone instruments due to their higher bandwidths.

Because every bus has strengths and weaknesses, no single bus is right for every application. However, for automated test applications, modular instruments based on standard PC I/O buses, PCI or PCI Express, provide the highest performance. Some systems, termed “hybrid systems,” also integrate multiple bus technologies together as shown in Figure 7.

PCI Express, the next evolution of the PCI bus, is the highest bandwidth – up to 4 GB/s – and lowest latency bus available. For example, PXI (PCI eXtensions for Instrumentation), an industry standard defining a rugged, high-performance test platform, uses PCI and PCI Express to provide the bus connectivity between the modular hardware and the PC.

Conclusion

When designing a test system it is important to understand how the architecture of traditional and modular instruments compares. Both instruments rely on similar measurement hardware to perform the A/D and D/A operations required for most measurements; however, how that measurement hardware is packaged greatly separates these two approaches.

While traditional instruments provide users with a standalone solution for quick, benchtop measurements, most of today’s automated test systems require the flexible, user-defined benefits of a modular instrumentation approach. In an ATE system, the measurements from the different instruments must be accurately correlated and synchronized together, which is enabled by the high-performance buses used in a modular instrumentation system. Along with the high-performance bus, the low cost approach to modular instrumentation helps reduce test system costs and increase flexibility, which has become critical for today’s test systems that must interface to a wide mix of devices that are growing in complexity.

For more detailed information on the features and benefits of a complete modular instrument system, please refer to the Understanding a Modular Instrumentation System for Automated Test whitepaper.

Relevant NI Products and Whitepapers

National Instruments, a leader in automated test, is committed to providing the hardware and software products engineers need to create these next generation test systems.

Software:

• NI TestStand Test Management Framework
• LabVIEW Graphical Programming Environment
• Signal Express Interactive Measurement Software
  

Hardware:

• Modular Instruments (Oscilloscopes, Multimeters, RF, Switching, and more)
• Multi-function Data Acquisition
• PXI System Components (Chassis and Controllers)
• Instrument Control (GPIB, USB, and LAN)
  

Whitepapers
NI offers a Designing Next Generation Test Systems Developers Guide. This guide is collection of whitepapers designed to help you develop test systems that lower your cost, increase your test throughput, and can scale with future requirements. To read the entire developers guide, you can: Download the PDF (90+ page) version or view the web-version of the Designing Next Generation Test Systems Developers Guide.

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