Note

This is a legacy module kept for backwards compatiblity with PyVISA < 1.5 and will be deprecated in future versions of PyVISA. You are strongly encouraged to switch to the new implementation.

# About the legacy visa module¶

Abstract

PyVISA enables you to control your measurement and test equipment – digital multimeters, motors, sensors and the like. This document covers the easy-to- use visa module of the PyVISA package. It implements control of measurement devices in a straightforward and convenient way. The design goal is to combine HTBasic’s simplicity with Python’s modern syntax and powerful set of libraries. PyVISA doesn’t implement VISA itself. Instead, PyVISA provides bindings to the VISA library (a DLL or “shared object” file). This library is usually shipped with your GPIB interface or software like LabVIEW . Alternatively, you can download it from your favourite equipment vendor (National Instruments, Agilent, etc).

It can be downloaded at the PyVISA project page. You can report bugs there, too. Additionally, I’m happy about feedback from people who’ve given it a try. So far, we have positive reports of various National Instruments GPIB adapters (connected through PCI, USB, and RS232), the Agilent 82357A, and SRS lock-in amplifiers, for both Windows and Linux. However, I’d be really surprised about negative reports anyway, due to the high abstraction level of PyVISA . As far as USB instruments are concerned, you must make sure that they act as ordinary USB devices and not as so-called HDI devices (like keyboard and mouse).

## An example¶

Let’s go in medias res and have a look at a simple example:

from pyvisa.legacy import visa

my_instrument = instrument("GPIB::14")
my_instrument.write("*IDN?")


This example already shows the two main design goals of PyVISA: preferring simplicity over generality, and doing it the object-oriented way.

Every instrument is represented in the source by an object instance. In this case, I have a GPIB instrument with instrument number 14, so I create the instance (i.e. variable) called my_instrument accordingly:

my_instrument = instrument("GPIB::14")


“GPIB::14” is the instrument’s resource name. See section VISA resource names for a short explanation of that. Then, I send the message “*IDN?” to the device, which is the standard GPIB message for “what are you?” or – in some cases – “what’s on your display at the moment?”:

my_instrument.write("*IDN?")


Finally, I print the instrument’s answer on the screen:

print(my_instrument.read())


## Example for serial (RS232) device¶

The only RS232 device in my lab is an old Oxford ITC4 temperature controller, which is connected through COM2 with my computer. The following code prints its self-identification on the screen:

from pyvisa.legacy import visa

itc4 = visa.instrument("COM2")
itc4.write("V")


Instead of separate write and read operations, you can do both with one ask() call. Thus, the above source code is equivalent to:

from pyvisa.legacy import visa

itc4 = visa.instrument("COM2")


It couldn’t be simpler. See section Serial devices for further information about serial devices.

## A more complex example¶

The following example shows how to use SCPI commands with a Keithley 2000 multimeter in order to measure 10 voltages. After having read them, the program calculates the average voltage and prints it on the screen.

I’ll explain the program step-by-step. First, we have to initialise the instrument:

from pyvisa.legacy import visa

keithley = visa.instrument("GPIB::12")
keithley.write("*rst; status:preset; *cls")


Here, we create the instrument variable keithley, which is used for all further operations on the instrument. Immediately after it, we send the initialisation and reset message to the instrument.

The next step is to write all the measurement parameters, in particular the interval time (500ms) and the number of readings (10) to the instrument. I won’t explain it in detail. Have a look at an SCPI and/or Keithley 2000 manual.

interval_in_ms = 500

keithley.write("status:measurement:enable 512; *sre 1")
keithley.write("trigger:source bus")
keithley.write("trigger:delay %f" % (interval_in_ms / 1000.0))

keithley.write("trace:feed sense1; feed:control next")


Okay, now the instrument is prepared to do the measurement. The next three lines make the instrument waiting for a trigger pulse, trigger it, and wait until it sends a “service request”:

keithley.write("initiate")
keithley.trigger()
keithley.wait_for_srq()


With sending the service request, the instrument tells us that the measurement has been finished and that the results are ready for transmission. We could read them with keithley.ask(“trace:data?”) however, then we’d get

NDCV-000.0004E+0,NDCV-000.0005E+0,NDCV-000.0004E+0,NDCV-000.0007E+0,
NDCV-000.0000E+0,NDCV-000.0007E+0,NDCV-000.0008E+0,NDCV-000.0004E+0,
NDCV-000.0002E+0,NDCV-000.0005E+0


which we would have to convert to a Python list of numbers. Fortunately, the ask_for_values() method does this work for us:

voltages = keithley.ask_for_values("trace:data?")
print "Average voltage: ", sum(voltages) / len(voltages)


Finally, we should reset the instrument’s data buffer and SRQ status register, so that it’s ready for a new run. Again, this is explained in detail in the instrument’s manual:

keithley.ask("status:measurement?")
keithley.write("trace:clear; feed:control next")


That’s it. 18 lines of lucid code. (Well, SCPI is awkward, but that’s another story.)

## VISA resource names¶

If you use the function instrument(), you must tell this function the VISA resource name of the instrument you want to connect to. Generally, it starts with the bus type, followed by a double colon ”::”, followed by the number within the bus. For example,

GPIB::10


denotes the GPIB instrument with the number 10. If you have two GPIB boards and the instrument is connected to board number 1, you must write

GPIB1::10


As for the bus, things like “GPIB”, “USB”, “ASRL” (for serial/parallel interface) are possible. So for connecting to an instrument at COM2, the resource name is

ASRL2


(Since only one instrument can be connected with one serial interface, there is no double colon parameter.) However, most VISA systems allow aliases such as “COM2” or “LPT1”. You may also add your own aliases.

The resource name is case-insensitive. It doesn’t matter whether you say “ASRL2” or “asrl2”. For further information, I have to refer you to a comprehensive VISA description like http://www.ni.com/pdf/manuals/370423a.pdf.

Footnotes

 [1] such as the “Measurement and Automation Center” by National Instruments
 [2] All flavours of binary data streams defined in IEEE488.2 are supported, i.e. those beginning with
#, where
is optional, and may also be “0”.
 [3] Of course, it’s highly advisable not to have installed another version of Python on your system before you install Enthought Python.
 [4] its name depends on the language of your Windows version