If you are using USB to communicate with the DMM, and it is USB RAW, then you will have to know specific information about USB communication with that device.
The USB
standard describes four types of pipes or endpoints: control, bulk, interrupt,
and isochronous. Each type of pipe transfers a different type of information.
Furthermore, any number of endpoints can be of any endpoint type. Think of an
endpoint as a communication socket.
NI-VISA supports three types of USB pipes: control, bulk, and interrupt. When
NI-VISA detects your USB instrument, it automatically scans your instrument for
the lowest available endpoint for each type.
NI-VISA includes four functions to transfer data through USB pipes. Before you
can communicate with your device using these functions, you need to set up the
communication protocol using the VISA USB attributes. The following list
describes the available functions.
Use VISA
USB Control In and VISA USB Control Out to transfer data using the control
pipe.
VISA Read
and VISA Write are used to communicate with USB Bulk Endpoints.
VISA
Write is used to communicate with USB Interrupt Out Endpoints.
If you
are using LabVIEW, VISA includes an
additional function to use the interrupt pipe: VISA Get USB Interrupt Data. In
the C API, you can do this by accessing the VI_ATTR_USB_RECV_INTR_SIZE and
VI_ATTR_USB_RECV_INTR_DATA attributes of the VI_EVENT_USB_INTR event object.
See the NI-VISA Programmer Reference Manual for more information
about VISA Events.
A key step in
communicating with a USB instrument is setting VISA as the driver linked to the
device. This is accomplished using the
Driver Development Wizard. The Driver Development Wizard automatically pulls the
manufacturer and device ID for connected USB devices and displays this
information to the user (VISA 5.0 and later). Older versions
of NI-VISA require the user to know the Device and Manufacturer ID to develop a
driver. You can also manually create a driver for USB devices if not connected.
The Driver Development Wizard will detect if the device is USBTMC or another
known USB device class, and warn the user that creating a driver for these
devices is not necessary.
Once you install this INF
file into the appropriate folder, plugging the USB device into the computer
will logically assign it to the VISA driver for handling. You can then use VISA VI’s or functions to
communicate with your USB instrument’s pipe.
You must know what sort of
endpoints (control, bulk, or interrupt) are available on the device, and how to
use those endpoints to communicate with your device.
That all being said (phew!), follow this tutorial: http://zone.ni.com/devzone/cda/tut/p/id/4478
Hope this helps!
Sean N
Applications Engineering Specialist - Semiconductor Test
National Instruments
