If you have been using PyVISA before version 1.5, you might want to read Migrating from PyVISA < 1.5.
Let’s go in medias res and have a look at a simple example:
>>> import visa >>> rm = visa.ResourceManager() >>> my_instrument = rm.get_instrument('GPIB::14') >>> my_instrument.write("*IDN?") >>> print(my_instrument.read())
This example already shows the two main design goals of PyVISA: preferring simplicity over generality, and doing it the object-oriented way.
Afer importing visa, we create a ResourceManager object. If called without arguments, PyVISA will try to find the VISA shared for you. You can check, the location of the shared library used simply by:
>>> print(rm) <ResourceManager('/path/to/visa.so')>
In some cases, PyVISA is not able to find the library for you resulting in an OSError. To fix it, find the library path yourself and pass it to the ResourceManager constructor. You can also specify it in a configuration file as discussed in Configuring PyVISA.
Once that you havea ResourceManager, you can access any instrument. 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 with “GPIB::14” is the instrument’s resource name. Notice that eventhough you have requeste an instrument, due to the resource name, get_instrument has given you an instance of GpibInstrument class (a subclass of the more generic instrument).
>>> print(my_instrument) <GpibInstrument('GPIB::14')>
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?”.
Listing connected instruments¶
The resource manager object allows you to list available resources:
>>> rm.list_resources() ['ASRL1', 'ASRL2']
or the most comprehensive list_resources_info which return a dict mapping resource name to a namedtuple containing information such as the interface type and the resource class.
Example for serial (RS232) device¶
There is no 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:
itc4 = rm.get_instrument("COM2") itc4.write("V") print(itc4.read())
Instead of separate write and read operations, you can do both with one ask() call. Thus, the above source code is equivalent to:
from visa import * itc4 = instrument("COM2") print(itc4.ask("V"))
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:
>>> keithley = rm.get_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 >>> number_of_readings = 10 >>> keithley.write("status:measurement:enable 512; *sre 1") >>> keithley.write("sample:count %d" % number_of_readings) >>> keithley.write("trigger:source bus") >>> keithley.write("trigger:delay %f" % (interval_in_ms / 1000.0)) >>> keithley.write("trace:points %d" % number_of_readings) >>> 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 get_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,
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
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
(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.