Physics 472, Graduate Laboratory DAQ with Matlab. Overview of data acquisition (DAQ) with GPIB

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1 1 Overview of data acquisition (DAQ) with GPIB The schematic below gives an idea of how the interfacing happens between Matlab, your computer and your lab devices via the GPIB bus. GPIB stands for General Purpose Interface Bus, and is used widely for laboratory instruments. It is by no means Matlab specific you can write C programs, use LabView, and many other languages to talk to instruments via the GPIB bus. GPIB goes by other names as well, including HPIB (or Hewlett Packard Interface Bus, after the original developers of the standard), and IEEE-488 because this is the official name for current the industry-standard form of GPIB. GPIB has been around for some time, as a much faster, parallel alternative to RS232 serial interfaces. A major vendor for GPIB products is National Instruments, who make LabView. Currently Ethernet and USB2-based interfacing are gaining in popularity over GPIB because of cost and speed advantages. Still, GPIB remains very widely used and serves as a good introduction to interfacing in general. At the hardware level: Bus is the generic name for a shared set of wires that may have more than one device connected to them, via which the various devices talk to one another or to a bus controller (which is the fancy name for the device on the bus that is running the show). Since the wires are shared, only one device at a time can drive them, i.e. set those wires to a high or low voltage; the other devices have to be hands-off, which is something that one can accomplish using transistors. Generally there are two buses involved, one of which is an address bus, the second of which is the data bus. The address bus is used to control which device should talk on the data bus. The data bus, just as its name implies, is used to send data back and forth between various devices or the controller. You may encounter many types of bus in your experimental work. The GPIB bus is one; another is the bus inside a desktop computer, the modern version of which is called the PCI bus. This is a set of shared wires and connector slots on the motherboard of your computer that allows the CPU and memory to talk to cards that you plug into those connectors. Typically, a computer has a special GPIB card plugged into the PCI bus; this allows the CPU and memory to communicate with that card, which in turn communicates with the instruments you plug into that card (via the GPIB bus). Here, we have GPIB to USB adapters, but the end result is the same. In order to talk to devices on your GPIB bus, Matlab needs to send commands through the GPIB board to those devices. Each device must already have been assigned a unique and known address on the GPIB bus. The various instruments (e.g. oscilloscope, function generator, etc.) listen to the GPIB bus, waiting for instructions. Since each instrument has different sets of capabilities and knobs you want to turn (e.g. to set frequency, or timebase, etc.), they each have a unique set of commands that the manufacturer has specified for them this means that in general you really need to read the manual to understand how to talk to a given instrument. Just because your program

2 2 works for, say, a LeCroy oscilloscope doesn t mean that it will work for a Tektronix oscilloscope; in fact, I ll guarantee it won t. Computer CPU GPIB Board GPIB Cables PCI Bus Oscilloscope Function Generator DMM/Scanner Other devices (up to 8 total, usually) Figure 1: A schematic of your computer s connection to instruments via GPIB.

3 3 In order to be able to talk back and forth on the GPIB bus, your various instruments must be properly addressed and your computer must know which address refers to which instrument. On the Windows machines, National Instruments, the vendor of the PCI bus GPIB card we use, supplies a tool to examine the GPIB bus. There is (or should be) an alias to it on your desktop called Measurement and Automation Tool. Open this up, and, following instructions, scan for instruments. Confirm that all of the instruments are on the GPIB bus. Note what the devices are called, i.e. does it say HP or HP 33120A, because your Matlab programs will refer to these names. Check that your devices are set to our preferred settings of: (Lecroy=1,HP=2,Keithley=3.) [Lecroy = oscilloscope, HP=function generator, Keithley = DMM/scanner; if you are using an SRS function generator, go ahead and call it SRS ]. Each of your instruments must have its GPIB address configured to match up with the value just entered in the computer s control panel settings. You can always find the addresses somewhere in that instrument s front-panel menu; it may be hard to find, so you might need to consult the manual for that instrument to find it. For your reference, here is how to locate some of them. LeCroy Oscilloscope 1. Press the Utilities button 2. Next find GPIB setup 3. turn the knob until you have set the address to 1. HP Function generator 1. shift (Menu) 2. E: I/O Menu 3. HPIB Addr 4. Select 2 Another good thing to know how to do is get rid of error messages: On the function generator, 1. Shift (Menu) 2. Sys Menu 3. Error 4. scroll down and find your error 5. press enter

4 4 Programming GPIB with Matlab. In Matlab, instrument control for GPIB is done using the Instrument Control Toolbox. This toolbox makes things fairly simple, but it does contain a fair amount of functionality (Interface Objects and Device Objects) that are more appropriate for more complex systems than we will deal with. For the most part, we will try to deal only with GBIP Objects and also a few tools to examine the hardware. Get started by reading the relevant sections of the Instrument Control Toolbox help. In the 2015b version, some of these are: Instrument Connection and Communication o Examining Your Hardware Resources Instrhwinfo - read carefully, and test at your computer. You should find the various instruments connected to your GPIB bus Test and Measurement Tool (tmtool) - test this, again finding the instruments on your bus. Interface-based Instrument Communication o GPIB Interface - read carefully, and test with your instruments. Troubleshooting -> Troubleshooting GPIB Interface - skim. (In the 2014b version, look for similar headings.) Now skip to the Controlling Instruments Using the GPIB section. Skim all of the GPIB Overview sections, then read the Creating a GPIB Object section. Next you will work through the Writing and Reading Data sections, replacing the specific instructions for use with the Tektronix scope they mention, with the tasks listed below. As you work and need to create GPIB objects for each instrument. You could name them gscope and gfg for the scope and function generator (FG) respectively. Do the following: 1. Set the FG to output a sine wave. The command string for a 5 khz, 2.0 V pp sine wave with 1V offset is: APPL:SIN 5.0E+3, 2.0,-1.0 Write a script that creates a sine wave based on the variables for the amplitude, frequency and offset. An example of a character array based the real variable x=3.20 is [ this variable x is equal to num2str(x)]. 2. Figure out how to measure a waveform from the scope. You will need to find the programming manual for the scope, and I will leave you to complete this step by reading that manual. You will find that initially you are only able to ready 512 bytes of data from the scope, which, since the data is returned as a character string, represents a limited amount of data. To read more data, increase the gscope.inputbuffersize to a larger value than the default for 512 bytes after you have created the gscope device object, but before you have opened it.

5 5 Measuring a transfer function. You will now put together all you have done so far this semester, and measure the transfer function of an RC lowpass filter using the computer to automate the measurement. First, build an RC lowpass filter with a 3dB frequency of approximately 1 khz. Readout the input and output signals on the two channels of the scope. Scan in frequencies to confirm that the circuit is functioning as a low-pass filter. Now write a Matlab program to automate the measurement of the transfer function. That is, in a loop over frequency, apply the two steps you coded above of setting a voltage at some frequency and then reading the input and output waves from the scope. Find the amplitudes of the two waves in Matlab (a good was is to used the var function -. The loop should range over frequencices from roughly 10 Hz to 100 khz. Here is one way to make a loop in matlab: frequencies = logspace(1,4,50); for nf=1:length(frequencies) freq=frequencies(nf); ** set voltage on function generator, read back signals and process *** end Note that you need to make sure that the waveform read from the scope is in range both in time and voltage you will have to adjust the sampling speed on the scope as you change the frequency, and you need to make sure the voltage is not outside the range of the scope (i.e., that it doesn t clip ). Make a graph of the transfer function, which is the ratio of output to input signal sizes, versus frequency. Try plotting this while changing both x and y axes linear and logarithmic (base 10 log). Generally, the choice for these type of transfer function measurements for both axes are base 10 logarithmic. As a bonus, find a way to measure the phase of the input and output signals, and make a graph of the phase difference versus frequency. Here the phase should be plotted linearly, as it has a small, bounded value.