3.14 Automating Measurements

3.14 Automating Measurements

Automating RF test and calibration systems can provide an engineer with the capability to perform a number of significant operations, namely report generation, data storage of test results, instrument synchronization and can speed up lengthy testing. Automated measurements require a PC, automation software, and interfacing between the instruments and PC.

The GPIB protocol is a well-established digital eight-bit parallel bus standard developed by HP for creating automated test sets with programmable instruments. With a little knowledge of an instrument's command set and a programming language, the user can automate a variety of test procedures. The steps for automating include establishing communication between the instrument and the computer, identifying the commands for controlling the instrument, and packaging the commands in a programming language to complete the automation.

A typical GPIB-based test setup comprises two or more GPIB devices (interface boards and/or instruments) connected using a shielded 24-conductor cable assembly. The plug and receptacle (male/female) connectors at each end of the cable allow devices to be linked in a linear or star configuration. Each instrument connected to the GPIB bus is assigned a unique address from 0 to 31. This standard lets any device on the bus talk or listen to any other device on the bus. The types of GPIB messages sent when devices are communicating are characterized as either device-dependent messages (also known as data messages or data) or interface messages (also known as command messages or commands).

Generally, data messages are programming instructions, machine status, measurement results and data files; command messages perform functions such as initializing the bus, setting device modes, and addressing and unaddressing devices. All GPIB devices can be categorized as controllers, talkers and/or listeners depending on their role in the test setup. The controller manages the flow of information on the GPIB by sending out commands that, among other things, ensure only one device is driving the bus at a time. Although there may be more than one controller in a system, only one is entitled to be the controller in charge at any one moment. There also must be a system controller defined for the GPIB setup. Typically, a computer is designated as the controller which essentially instructs a talker to talk (place data on the bus) and a listener to listen (read the data).

When developing an application with GPIB, a computer program is first written with simple trial-and-error steps. Establishing communication between the instrument and the computer is a necessary first step whereby users send commands to the instrument and read the responses by means of a GPIB board in the computer and associated driver software. Users also can monitor the status of the GPIB operation with the Interface Bus Interactive Control (IBIC) utility. This utility allows the user to uncover incorrect addresses, power-off conditions and other simple problems without writing a program.

An instrument driver routine is often written for instruments used in numerous applications or by many users in a test facility. The instrument driver contains all the device-specific commands for that instrument in a program with a graphical user interface (GUI).

Reference:

Michael Griffith and Hall T. Martin, National Instruments

National Instruments website: http://www.ni.com/labview/test/ 


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