We assume that you have some background - however sparse - in programming.  In this lesson, we will examine how to communicate with an instrument using LabVIEW and a GPIB (IEEE-488) connection.  We will focus on the Fluke Hydra Data Acquisition Unit (DAU), but what is covered here is applicable to many other instruments.  Note that LabVIEW is a copyrighted name owned by the National Instruments Corporation, and that is the corporation that sells LabVIEW.  It is the most widely used programming environment for instrument control although there is considerable instrument programming done in C/C++ and Visual Basic.  LabVIEW is a visual programming environment (which means that you do not write sequential lines of code) and seems to be uniquely matched to the ways of thinking and solving problems that engineers and scientists commonly employ.

        The first step in instrument programming is to reset the instrument under program control in LabVIEW.  What you need for that process is the following.

• First, you need to start LabVIEW and generate a new "vi".  LabVIEW files often have a "vi" extension, and that stands for "Virtual Instrument".  You are about to embark on developing a virtual instrument that uses the Hydra DAU to take the actual measurements.  What you write in LabVIEW will be a program that controls that process.
• You need to know the GPIB address for the instrument.  In many cases - especially with more recent instruments - the GPIB address will flash for a moment as the instrument cycles through its startup procedure.  In the case of the Hydra, you need to set that address.  Within our department, the standard address is 3, and you will find most programs use that address.
• Once you have that taken care of, the Hydra should be ready to be accessed from within LabVIEW.

        In the lab, you will find that the numbered terminals for the Hydra have been brought out to a connector block attached to the top of the Hydra.  Those terminals are shown below in the photo of the Hydra and we are referring to the orange terminal blocks there.

        In the example in this note, we will be taking a temperature measurement from Channel 1 of the Hydra.  That means that you should connect the thermocouple leads to the Hydra.

• Attach the red lead to terminal #22.
• Attach the other lead to terminal #21.

        Taking a temperature measurement is more complex than just resetting the instrument.  The steps you need to take are as follows (assuming that the instrument has been reset).  Here's the diagram for the vi.

Here's what you do in this vi.

• Send the following commands using the GPIB Write block in LabView.  In the diagram below, there are three GPIB Write blocks.  Note that each of them uses the same address (3) and the same mode (3) as in the reset vi above.  Here are the commands that are sent to the instrument in each of the GPIB Write blocks.
• FUNC 1,TEMP,K - which sets the function on Channel 1 to be TEMP (TEMPerature measurement) using a type K thermocouple.
• *TRG - which triggers a measurement when the instrument receives this command.
• LAST? 1 - which requests the instrument to prepare the last measurement on Channel 1 for transmission to the computer.
• In LabView you sequence these commands by taking the error signal from each block and feeding it to the next block.  See the diagram reproduced below.
• After setting up the instrument, taking a measurement and preparing the data, you must get the instrument to send the string to the computer.  In LabView you use a GPIB Read block.  It's found in the same general place as the GPIB Write block.

        The vi above will take a single temperature measurement once the instrument has been reset.  That allows you to put the temperature mesurement inside a loop and put the reset outside the loop.  If you use this in an application where speed is critical, you might also want to consider putting the GPIB Write block that sets the channel and the measurement type outside the loop, separating it from the rest of the operations above.