Reading and Writing values

Some instruments allow to transfer to and from the computer larger datasets with a single query. A typical example is an oscilloscope, which you can query for the whole voltage trace. Or an arbitrary wave generator to which you have to transfer the function you want to generate.

Basically, data like this can be transferred in two ways: in ASCII form (slow, but human readable) and binary (fast, but more difficult to debug).

PyVISA Message Based Resources have different methods for this called read_ascii_values(), query_ascii_values() and read_binary_values(), query_binary_values().

Reading ASCII values

If your oscilloscope (open in the variable inst) has been configured to transfer data in ASCII when the CURV? command is issued, you can just query the values like this:

>>> values = inst.query_ascii_values('CURV?')

values will be list containing the values from the device.

In many cases you do not want a list but rather a different container type such as a numpy.array. You can of course cast the data afterwards like this:

>>> values = np.array(inst.query_ascii_values('CURV?'))

but sometimes it is much more efficient to avoid the intermediate list, and in this case you can just specify the container type in the query:

>>> values = inst.query_ascii_values('CURV?', container=numpy.array)

In container, you can have any callable/type that takes an iterable.

Note

When using numpy.array or numpy.ndarray, PyVISA will use numpy routines to optimize the conversion by avoiding the use of an intermediate representation.

Some devices transfer data in ASCII but not as decimal numbers but rather hex or oct. Or you might want to receive an array of strings. In that case you can specify a converter. For example, if you expect to receive integers as hex:

>>> values = inst.query_ascii_values('CURV?', converter='x')

converter can be one of the Python string formatting codes. But you can also specify a callable that takes a single argument if needed. The default converter is 'f'.

Finally, some devices might return the values separated in an uncommon way. For example if the returned values are separated by a '$' you can do the following call:

>>> values = inst.query_ascii_values('CURV?', separator='$')

You can provide a function to takes a string and returns an iterable. Default value for the separator is ',' (comma)

Reading binary values

If your oscilloscope (open in the variable inst) has been configured to transfer data in BINARY when the CURV? command is issued, you need to know which type datatype (e.g. uint8, int8, single, double, etc) is being used. PyVISA use the same naming convention as the struct module.

You also need to know the endianness. PyVISA assumes little-endian as default. If you have doubles d in big endian the call will be:

>>> values = inst.query_binary_values('CURV?', datatype='d', is_big_endian=True)

You can also specify the output container type, just as it was shown before.

By default, PyVISA will assume that the data block is formatted according to the IEEE convention. If your instrument uses HP data block you can pass header_fmt='hp' to read_binary_values. If your instrument does not use any header for the data simply header_fmt='empty'.

By default PyVISA assumes, that the instrument will add the termination character at the end of the data block and actually makes sure it reads it to avoid issues. This behavior fits well a number of devices. However some devices omit the termination character, in which cases the operation will timeout. In this situation, first makes sure you can actually read from the instrument by reading the answer using the read_raw function (you may need to call it multiple time), and check that the advertized length of the block match what you get from your instrument (plus the header). If it is so, then you can safely pass expect_termination=False, and PyVISA will not look for a termination character at the end of the message.

If you can read without any problem from your instrument, but cannot retrieve the full message when using this method (VI_ERROR_CONN_LOST, VI_ERROR_INV_SETUP, or Python simply crashes), try passing different values for chunk_size``(the default is 20*1024). The underlying mechanism for this issue is not clear but changing ``chunk_size has been used to work around it. Note that using larger chunk sizes for large transfer may result in a speed up of the transfer.

In some cases, the instrument may use a protocol that does not indicate how many bytes will be transferred. The Keithley 2000 for example always return the full buffer whose size is reported by the ‘trace:points?’ command. Since a binary block may contain the termination character, PyVISA need to know how many bytes to expect. For those case, you can pass the expected number of points using the data_points keyword argument. The number of bytes will be inferred from the datatype of the block.

Finally if you are reading a file for example and simply want to extract a bytes object, you can use the "s" datatype and pass bytes as container.

Writing ASCII values

To upload a function shape to arbitrary wave generator, the command might be WLISt:WAVeform:DATA <waveform name>,<function data> where <waveform name> tells the device under which name to store the data.

>>> values = list(range(100))
>>> inst.write_ascii_values('WLISt:WAVeform:DATA somename,', values)

Again, you can specify the converter code.

>>> inst.write_ascii_values('WLISt:WAVeform:DATA somename,', values, converter='x')

converter can be one of the Python string formatting codes. But you can also specify a callable that takes a single argument if needed. The default converter is 'f'.

The separator can also be specified just like in query_ascii_values.

>>> inst.write_ascii_values('WLISt:WAVeform:DATA somename,', values, converter='x', separator='$')

You can provide a function to takes a iterable and returns an string. Default value for the separator is ',' (comma)

Writing binary values

To upload a function shape to arbitrary wave generator, the command might be WLISt:WAVeform:DATA <waveform name>,<function data> where <waveform name> tells the device under which name to store the data.

>>> values = list(range(100))
>>> inst.write_binary_values('WLISt:WAVeform:DATA somename,', values)

Again you can specify the datatype and endianness.

>>> inst.write_binary_values('WLISt:WAVeform:DATA somename,', values, datatype='d', is_big_endian=False)

If your data are already in a bytes object you can use the "s" format.

When things are not what they should be

PyVISA provides an easy way to transfer data from and to the device. The methods described above work fine for 99% of the cases but there is always a particular device that do not follow any of the standard protocols and is so different that cannot be adapted with the arguments provided above.

In those cases, you need to get the data:

>>> inst.write('CURV?')
>>> data = inst.read_raw()

and then you need to implement the logic to parse it.

Alternatively if the read_raw call fails you can try to read just a few bytes using:

>>> inst.write('CURV?')
>>> data = inst.read_bytes(1)

If this call fails it may mean that your instrument did not answer, either because it needs more time or because your first instruction was not understood.