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 vpp43 module¶

This module vpp43 is a cautious yet thorough adaption of the VISA specification for Python. The “textual languages” VISA specification can’t be implemented as is because Python is rather different from C and Visual Basic, most notably because of lacking call-by-reference. The second important difference are strings: In C they are null-terminated whereas Python doesn’t have this constraint.

The slightly odd name vpp43 for this module derives from the necessity to make (name)space for the visa module that is supposed to realise the actual high-level VISA access in Python. The VXIplug&play Systems Alliance used to maintain the VISA specifications, and, although today the IVI foundation is responsible for this task, the files are still called vpp43.doc etc. So I thought vpp43 was an appropriate name.

You may wonder why I did choose new names for all routines. I did so because Python has its own naming guidelines, and because it shows that the routines had to be adapted. However, I didn’t change them really: Every routine is a 1:1 counterpart. By calling them from C, you could even create a C-based VISA implementation with the original function signatures and semantics. Moreover, the new names are mere expansions of the original ones.

## Connecting to the VISA shared object¶

vpp43 tries to find the VISA library for itself. On Windows, this is not a big problem. visa32.dll must be in your PATH. If it isn’t, move it there or expand your PATH.

However, on Linux you may need to give the explicit path to the shared object file. You do so by saying for example:

from pyvisa.legacy import vpp43


By default, vpp43 looks for the library in /usr/local/vxipnp/linux/bin/libvisa.so.7. Please pay attention to the fact that the library must have been successfully loaded before any VISA call is made.

Alternatively, you can tell PyVISA so by creating a file ~/.pyvisarc. This has the format of an INI file. For example, if the library is at /usr/lib/libvisa.so.7, the file .pyvisarc must contain the following:

[Paths]

VISA library: /usr/lib/libvisa.so.7


Please note that [Paths] is treated case-sensitively.

You can define a site-wide configuration file at /usr/share/pyvisa/.pyvisarc. (It may also be /usr/local/... depending on the location of your Python.)

## Diagnostics¶

This module can raise a couple of vpp43-specific exceptions.

Name: VisaIOError This is an error of the underlying VISA library, as described in table 3.3.1 in the VISA specification for textual languages. The exception member error_code contains the (always negative) VISA error number, as listed in that table. VisaIOWarning This is a warning of the underlying VISA library, as described in table 3.3.1 in the VISA specification for textual languages. The exception member completion_code contains the (always positive) VISA completion number, as listed in that table. Normally you don’t see these warnings. You can turn them into exceptions with: import warnings warnings.filterwarnings("error")  Consult the description of the warnings package for further information.
Name: TypeError The current implementation of printf_, scanf_, sprintf_, sscanf_, and queryf_ have the limitation that only integers, floats, and strings are allowed as types for the arbitrary arguments. Additionally, only format string directives for C longs, C doubles, and C strings are allowed to use, albeit not checked. However, if you pass a list or a unicode string, you get this exception. The same exception is raised if an unsupported type is passed as user handle to install_handler_. See there for a list of supported types. UnknownHandler Raised if an unknown handler/user_handle pair is passed to uninstall_handler_. In particular, you must save the user handle returned by install_handler_ in order to pass it to uninstall_handler.

Moreover, this module may pass exceptions generated by ctypes. This may be because you’ve passed a wrong type to a function, or that the VISA library file was not found, but it may also mean a bug in vpp43 itself. So if you don’t see why the exception was raised, contact the current maintainers of PyVISA.