Key takeaways:
- Programmatic as well as non-programmatic instrument control are essential for automating tests in productive, repeatable, and convenient ways.
- Instrument control must be able to work with a variety of physical and programmatic interfaces.
- For automation of measurements and tests, instrument control must support high data rates, fast settling speeds, and precise readings according to specifications from instrument datasheets.
Critical industries like defense, telecom, automotive, semiconductor, and avionics rely heavily on stringent testing of their electronic equipment at all levels, from the tiniest integrated circuit to heavy high-voltage electrical equipment.
The testing in each of these industries goes both wide and deep. But setting up and conducting such stringent testing manually is just not practical. Instead, they rely heavily on the automation of measurements and tests for efficiency and accuracy.
Automation, in turn, relies heavily on robust instrument control designs and workflows. In this article, get an overview of instrument control aspects in the field of electronic test and measurement.
What is instrument control in electronic test and measurement?
Instrument control is the use of specialized computer software that can communicate remotely with electronics test and measurement (T&M) instruments to enable the following capabilities:
- fine-grained configuration of each instrument’s specialized functions
- automated measurements by individual instruments
- automation of test sequences by individual instruments
- orchestration of complex test sequences across many instruments
The classes of instruments that can be controlled include:
Besides these primary capabilities, instrument control enables many convenience, efficiency, and performance improvements in T&M workflows.
We’ll explain the technologies underlying instrument control and explain the improvements they enable in greater detail next.
What are the technologies and protocols used in remote instrument control?
Fig 1. Instrument control physical and programming interfaces
The illustration above shows the physical interfaces, operating system components, application programming interfaces (APIs), and application languages frequently used in instrument control. We’ll elaborate on each one in the following sections.
Physical interfaces
Different physical interfaces between computers and instruments have been developed or adopted over time. Some are T&M-specific while others are standard computing interfaces. They differ in their throughput, scalability, location, convenience, and productivity of instrument control. We review the popular interfaces below.
General purpose interface bus (GPIB)
Fig 2. Three Keysight 3458A digital multimeters on a single GPIB
The GPIB is a T&M-specific 24-bit parallel, digital, external interface that started as a proprietary connection and was later standardized by the International Organization for Standards (ISO) 488.
Its capabilities include:
- connecting up to 15 instruments on a single bus
- total bus length of about 65.5 feet with up to 6.5 feet between any two instruments
- maximum data rate of about one megabyte per second (MB/s)
Three instruments on a single GPIB are shown in the image above.
LAN/Ethernet and LAN extensions for instrumentation (LXI)
The local area network (LAN) Ethernet interface is a popular external interface for wired networking. Unlike GPIB or USB, multiple instruments can’t be daisy-chained on a single Ethernet cable. Instead, a separate Ethernet hub is used to connect many devices to a single computer port.
Ethernet’s characteristics make it suitable for applications that involve a lot of mobility and data transfer with:
- maximum distance of 300-plus feet
- data rates of 100 MB/s to 10 gigabytes per second (GB/s)
The data and instructions between the computer and instruments are communicated over transmission control protocol (TCP) and internet protocol (IP).
LXI is an external interface that extends Ethernet with T&M-specific capabilities for timing, synchronization, event messaging, and security.
Universal serial bus (USB) and USB test and measurement class (USBTMC)
USB is among the most convenient external interfaces because it’s available on most computing devices. Its capabilities include:
- daisy chaining up to 128 devices — but in practice this is rare
- maximum bus length of about 16-20 feet
- very high data rates ranging between 480 MB/s and 20 GB/s
The USB test and measurement class (USBTMC) is a standard device category that defines the endpoints that enable instruments to identify as T&M devices.
Peripheral connect interface (PCI) and PCI extensions for instrumentation (PXI)
Fig 3. An M9352A PXI hybrid amplifier module
PCI, PCI express (PCIe), PXI, and PXI express (PXIe) are internal interfaces that are preferred for pluggable devices in modular systems like DAQ and ATE. PCI and PCIe are standard computer interfaces while PXI is their T&M-specific variant.
Operating system components
The operating system-level components provide input-output abstractions and data translations for the API layer above them.
Instrument drivers
Instrument manufacturers publish drivers for some of the more complex instruments. These drivers convert high-level API calls to instrument-specific commands and configurations to help shield the applications from low-level details of the device.
Direct Input/Output (IO)
Some instruments don’t require any abstraction or translation between the APIs and the physical interfaces. For such instruments, direct IO is used.
Application programming interfaces
Several application programming interfaces (APIs) in the form of software libraries have been designed to shield end-user applications from having to know the details of the above low-level physical interfaces.
In other words, these enable applications to talk uniformly to any instrument no matter how it’s physically connected to the computer. These logical interfaces are explained below.
SCPI
Standard Commands for Programmable Instruments (SCPI) commands are instrument-specific instructions for taking measurements and running test sequences.
Each instrument supports different SCPI command sets for its specific capabilities. Some commands are standardized for common instrument classes like signal analyzers. However, instrument manufacturers are allowed to define custom commands too.
A complex sequence of SCPI commands is shown below.
Fig 4. An SCPI script to measure the output of a power supply
Virtual instrument software architecture (VISA) and PyVISA
VISA is a simple, bus-independent, standardized API for instrument control. Applications can use it to control any VISA-compatible instrument.
VISA terms instruments and instrument capabilities as resources. It provides instrument control services like:
- Lifecycle management: VISA enables systematic initialization and frees up resources to help the instruments optimize their instrument control capabilities — for example, the “viClose” API.
- Configuration management: Fine-grained instrument configuration is possible through the “viGetAttribute” and “viSetAttribute” APIs.
- Measurement control: Measurement and test capabilities are instrument-specific and exercised by another API called the standard commands for programmable instruments (SCPI). Once those APIs have obtained the data, generic VISA APIs like “viRead,” “viWrite,” and memory operations enable the data to be transferred between the instrument and the computer.
- Operation control: Asynchronous jobs can be started and terminated for measurement or test sequences that take a long time.
- Access control: VISA also provides access control to safeguard access and use of instruments in sensitive industries like defense.
The main disadvantage of VISA is that it’s a relatively low-level API that requires the application to be aware of instrument-specific attributes and commands.
PyVISA is a wrapper component to use VISA from Python software. It enables Python-based command-line, desktop, and web applications to provide custom user interfaces for instrument control.
Interchangeable virtual instruments (IVI)
IVI aims to avoid instrument-specific command sets (such as SCPI) and provide uniform APIs for different instruments of the same class. For example, all oscilloscopes can be controlled by the same API regardless of their brands. It also specifies IVI instrument driver interfaces.
This uniformity reduces the need to deal with the specifics of each instrument’s command language. The APIs are also more syntactically and semantically consistent compared to the text-based SCPI commands.
Applications
You can implement customized instrument control applications in languages like Python, C/C++, and .NET languages like C# and VB.NET. They send commands and receive data using the APIs.
How does software facilitate instrument control in laboratories and manufacturing?
Instrument control capabilities are greatly affected by the design and deployment aspects of all the software components. Here are some ways in which software affects or facilitates instrument control:
- Interoperability with different instruments: The IVI and VISA APIs enable applications to work with any class of instrument from any manufacturer. They also enable the orchestration of test sequences across multiple instruments.
- Interoperability with popular software: Popular T&M software like Keysight PathWave BenchVue, MathWorks MATLAB, Simulink, and LabVIEW can use instrument control to integrate multiple instruments into their user interfaces and workflows. Using instrument control, data can be transferred to accessible software like Microsoft Excel for quick data analysis and visualization.
- Programmatic interfaces: The interfaces above enable programmatic access, scripting, automation, and visualization of measurements and tests.
- User-friendly graphical interfaces: The same interfaces are employed by end-user applications to implement friendly graphical user interfaces (GUIs) for non-programmers. Such users can use simple point-and-click actions to automate and visualize complex measurements and tests.
- Real-time measurements: Automated instrument control is particularly helpful when repeated measurements are required in real time with very low latency.
- Remote and distributed testing: Software enables test engineers in different locations to orchestrate complex test operations through the use of remote and cloud testing infrastructure.
- Security: Software facilitates security of data and access for T&M instrument control in sensitive industries like aerospace and defense where data theft, malware, and targeted attacks can have disastrous consequences.
How does instrument control contribute to automation in laboratory and industrial processes?
Automation is one of the biggest benefits and motivations of instrument control. It enables the following capabilities:
- Automated measurements: The type, timing, and frequency of measurements can be precisely specified.
- Automation of functional and other tests: Instrument control facilitates system, functional, in-circuit, performance, and conformance tests in different industries like automotive, defense, and semiconductor.
- Orchestration of complex test sequences: System tests often involve intricate sequences with precise timings. For example, a power supply must source a specific current waveform for a precise duration to a device under test (DUT). Simultaneously, a multimeter and an oscilloscope are waiting on triggers to start measuring until a stop signal is received. For orchestrating such intricate sequences, instrument control is essential.
- Repeatability: Instrument control promotes the repeatability of test sequences.
- Remote monitoring: Instrument control improves efficiency and productivity by enabling remote monitoring of instruments and results.
- Safety: For use cases like high-voltage tests and electric vehicle battery monitoring, remote instrument control is essential for the physical safety of engineers and technicians.
How does instrument control improve reliability and accuracy?
Instrument control improves the reliability and accuracy of T&M workflows as follows:
- Precise configuration: Precise and fine-grained control over the instrument configuration allows accurate measurements and avoids the variability of manual control.
- Control over accuracy factors: Instrument control enables the checking of conditions required for high accuracy. For example, instrument control can automatically verify the calibration status prior to a measurement or test and alert the engineer if calibration is required.
- Improved reliability: Complex functional and performance test sequences can be automated and run frequently and after every change in the device’s lifetime.
Keysight’s instrument control solutions
Keysight publishes a powerful set of applications and libraries for instrument control. We review them in the sections below.
Keysight Command Expert
Fig 5. Keysight Command Expert
The Keysight Command Expert is an easy-to-use instrument control GUI application that engineers can use to run SCPI commands and scripts. It provides productivity and convenience features like command search, reference documentation for every command, syntax checks, and export of SCPI scripts to Python, MATLAB, C#, VB.NET, and C/C++.
Command Expert supports a wide variety of Keysight instruments. The application automatically downloads the appropriate SCPI command sets based on the instruments you connect.
Keysight IO Libraries Suite
The Keysight IO Libraries Suite is a collection of reusable software libraries and drivers for instrument control over the entire set of Keysight instruments as well as third-party devices. It implements VISA and IVI API support on Windows and Linux operating systems.
PathWave BenchVue
Fig 6. Keysight PathWave BenchVue
Unlike the above developer-oriented software, PathWave BenchVue is a full-fledged T&M instrument control platform with a large number of integrated instrument-specific GUIapps that provide advanced instrument control and automated test sequences with user-friendly interfaces.
Keysight VEE Pro
Keysight VEE Pro is an easy visual programming tool for creating test and measurement sequences graphically and automating them.
Get unprecedented instrument control with Keysight software
In this article, we explored the fundamentals and technologies that enable instrument control and unlock a variety of advanced use cases through it.
Contact us for expert insights on automating your measurement and testing requirements.