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Reading and Writing ASCII Data Using VISA

This example explores ASCII read and write operations with a VISA object. The instrument used was a Tektronix® TDS 210 oscilloscope.

The VISA object supports seven interfaces: serial, GPIB, VXI, GPIB-VXI, TCPIP, USB, and RSIB. This example explores ASCII read and write operations using a VISA-GPIB object. However, ASCII read and write operations for VISA-GPIB, VISA-VXI, VISA-GPIB-VXI, VISA-TCPIP, VISA-SERIAL, and VISA-USB objects are identical to each other. Therefore, you can use the same commands. The only difference is the resource name specified in the VISA constructor.

ASCII read and write operations for the VISA-serial object are identical to ASCII read and write operations for the serial port object. Therefore, to learn how to perform ASCII read and write operations for the VISA-serial object, you should refer to the Serial Port ASCII Read/Write tutorial.

ASCII read and write operations for the VISA-RSIB object are identical to the ASCII read and write operations for the VISA-GPIB, VISA-VXI, VISA-GPIB-VXI, VISA-TCPIP, and VISA-USB objects, except the VISA-RSIB object does not support the EOSCharCode and EOSMode properties.

These functions are used when reading and writing text:

FunctionPurpose
fprintfWrite text to an instrument.
fscanfRead data from an instrument and format as text.

These properties are associated with reading and writing text:

PropertyPurpose
ValuesReceivedSpecifies the total number of values read from the instrument.
ValuesSentSpecifies the total number of values sent to the instrument.
InputBufferSizeSpecifies the total number of bytes that can be queued in the input buffer at one time.
OutputBufferSizeSpecifies the total number of bytes that can be queued in the output buffer at one time.
EOSModeConfigures the End-Of-String termination mode.
EOSCharCodeSpecifies the End-Of-String terminator.
EOIModeEnables or disables the assertion of the EOI mode at the end of a write operation.

Configuring and Connecting to the Instrument

You need to create a VISA-GPIB object. In this example, an object is created using the ni driver and the VISA resource string shown below.

v = visa('ni', 'GPIB0::2::INSTR');

Before you can perform a read or write operation, you must connect the VISA-GPIB object to the instrument with the fopen function.

fopen(v);

If the object was successfully connected, its Status property is automatically configured to open.

v.Status
ans = 
    open

Writing ASCII Data

You use the fprintf function to write ASCII data to the instrument. For example, the'Display:Contrast' command will change the display contrast of the oscilloscope.

fprintf(v, 'Display:Contrast 45');

By default, the fprintf function operates in a synchronous mode. This means that fprintf blocks the MATLAB® command line until one of the following occurs:

  • All the data is written

  • A timeout occurs as specified by the Timeout property

By default the fprintf function writes ASCII data using the %s\n format. You can also specify the format of the command written by providing a third input argument to fprintf. The accepted format conversion characters include: d, i, o, u, x, X, f, e, E, g, G, c, and s. For example:

fprintf(v, '%s', 'Display:Contrast 45');

ASCII Write Properties

OutputBufferSize

The OutputBufferSize property specifies the maximum number of bytes that can be written to the instrument at once. By default, OutputBufferSize is 512.

v.OutputBufferSize
ans = 
    512

If the command specified in fprintf contains more than 512 bytes, an error is returned and no data is written to the instrument.

EOIMode, EOSMode, and EOSCharCode

By default, the End or Identify (EOI) line is asserted when the last byte is written to the instrument. This behavior is controlled by the EOIMode property. When EOIMode is set to on, the EOI line is asserted when the last byte is written to the instrument. When EOIMode is set to off, the EOI line is not asserted when the last byte is written to the instrument.

All occurrences of \n in the command written to the instrument are replaced with the EOSCharCode property value if EOSMode is set to write or read&write.

ValuesSent

The ValuesSent property is updated by the number of values written to the instrument. Note that by default EOSMode is set to none. Therefore, EOSCharCode is not sent as the last byte of the write.

fprintf(v, 'Display:Contrast 45');
v.ValuesSent
ans = 
    57

Clear any data in the input buffer before moving to the next step.

flushinput(v);

Reading ASCII Data

You use the fscanf function to read ASCII data from the instrument. For example, the oscilloscope command 'Display:Contrast?' returns the oscilloscope's display contrast:

fprintf(v, 'Display:Contrast?');
data = fscanf(v)

data =

    45

fscanf blocks until one of the following occurs:

  • The EOI line is asserted

  • The terminator is received as specified by the EOSCharCode property

  • A timeout occurs as specified by the Timeout property

  • The input buffer is filled

  • The specified number of values is received

By default, the fscanf function reads data using the '%c' format. You can also specify the format of the data read by providing a second input argument to fscanf. The accepted format conversion characters include: d, i, o, u, x, X, f, e, E, g, G, c, and s. For example, the following command will return the voltage as a decimal:

fprintf(v, 'Display:Contrast?');
data = fscanf(v, '%d')

data =

    45

isnumeric(data)

ans =

    1

ASCII Read Properties

InputBufferSize

The InputBufferSize property specifies the maximum number of bytes you can read from the instrument. By default, InputBufferSize is 512.

v.InputBufferSize
ans = 
    512

ValuesReceived

The ValuesReceived property indicates the total number of values read from the instrument. Note the last value received is a linefeed.

fprintf(v, 'Display:Contrast?');
data = fscanf(v)

data =

    45

v.ValuesReceived

ans =

     9

EOSMode and EOSCharCode

To terminate the data transfer based on receiving EOSCharCode, you should set the EOSMode property to read or read&write and the EOSCharCode property to the ASCII code for which the read operation should terminate. For example, if you set EOSMode to read and EOSCharCode to 10, then one of the ways that the read terminates is when the linefeed character is received.

The standard response to the vertical gain query is in scientific notation.

fprintf(v, 'CH1:Scale?')
data = fscanf(v)

data =

    1.0E0

Now configure the VISA-GPIB object to terminate the read operation when the 'E' character is received. The first read terminates when the 'E' character is received.

set(v, 'EOSMode', 'read')
set(v, 'EOSCharCode', double('E'))
fprintf(v, 'CH1:Scale?')
data = fscanf(v)

data =

1.0E

If you perform a second read operation, it terminates when the EOI line is asserted.

data = fscanf(v)

data =

    0

Cleanup

If you are finished with the VISA-GPIB object, disconnect it from the instrument, remove it from memory, and remove it from the workspace.

fclose(v);
delete(v);
clear v
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