Working with the Serial Port Interface :: Controlling Instruments Using VISA (Instrument Control Toolbox™)

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Working with the Serial Port Interface

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Understanding the Serial Port Creating a VISA-Serial Object Configuring Communication Settings

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Understanding the Serial Port

The serial port interface is supported through a VISA-serial object. The features associated with a VISA-serial object are similar to the features associated with a serial port object. Therefore, only functions and properties that are unique to VISA's serial port interface are discussed in this section.

Refer to Controlling Instruments Using the Serial Port, to learn about writing and reading text and binary data, using events and callbacks, using serial port control lines, and so on.

Note The VISA-serial object does not support the serialbreak function, the BreakInterruptFcn property, and the PinStatusFcn property.

Creating a VISA-Serial Object

You create a VISA-serial object with the visa function. Each VISA-serial object is associated with an instrument connected to a serial port on your computer.

visa requires the vendor name and the resource name as input arguments. The vendor name can be agilent, ni, or tek. The resource name consists of the name of the serial port connected to your instrument. You can find the VISA-serial resource name for a given instrument with the configuration tool provided by your vendor, or with the instrhwinfo function. (In place of the resource name, you can use an alias as defined with your VISA vendor configuration tool.) As described in Connecting to the Instrument, you can also configure property values during object creation.

Before you create a VISA object, you must find the instrument in the appropriate vendor VISA explorer. When you find the instrument configured, note the resource string and create the object using that information. For example, to create a VISA-serial object that is associated with the COM1 port, and that uses National Instruments VISA,

vs = visa('ni','ASRL1::INSTR');

The VISA-serial object vs now exists in the MATLAB workspace.

To open a connection with the instrument, type:

fopen (vs);

You can then display the class of vs with the whos command.

whos vs
  Name      Size         Bytes  Class

  vs        1x1            888  visa object

Grand total is 18 elements using 888 bytes

After you create the VISA-serial object, the properties listed below are automatically assigned values. These properties provide descriptive information about the object based on its class type and address information.

VISA-Serial Descriptive Properties

Property Name	Description
`Name`	Specify a descriptive name for the VISA-serial object.
`Port`	Indicate the serial port name.
`RsrcName`	Indicate the resource name for a VISA instrument.
`Type`	Indicate the object type.

You can display the values of these properties for vs with the get function.

get(vs,{'Name','Port','RsrcName','Type'})
ans = 
'VISA-Serial-ASRL1'    'ASRL1'    'ASRL1::INSTR'    'visa-serial'

VISA-Serial Object Display

The VISA-serial object provides you with a convenient display that summarizes important address and state information. You can invoke the display summary these three ways:

Type the VISA-serial object at the command line.
Exclude the semicolon when creating a VISA-serial object.
Exclude the semicolon when configuring properties using the dot notation.

You can also display summary information via the Workspace browser by right-clicking an instrument object and selecting Display Summary from the context menu.

The display summary for the VISA-serial object vs is given below.

VISA-Serial Object Using NI Adaptor : VISA-Serial-ASRL1

Communication Settings 
   Port:               ASRL1
   BaudRate:           9600
   Terminator:         'LF'

Communication State 
   Status:             closed
   RecordStatus:       off

Read/Write State  
   TransferStatus:     idle
   BytesAvailable:     0
   ValuesReceived:     0
   ValuesSent:         0

Configuring Communication Settings

Before you can write or read data, both the VISA-serial object and the instrument must have identical communication settings. Configuring serial port communications involves specifying values for properties that control the baud rate and the Serial Data Format. These properties are given below.

VISA-Serial Communication Properties

Property Name	Description
`BaudRate`	Specify the rate at which bits are transmitted.
`DataBits`	Specify the number of data bits to transmit.
`Parity`	Specify the type of parity checking.
`StopBits`	Specify the number of bits used to indicate the end of a byte.
`Terminator`	Specify the character used to terminate commands written to the instrument.

Refer to your instrument documentation for an explanation of its supported communication settings. Note that the valid values for StopBits are 1 and 2 and the valid values for Terminator do not include CR/LF and LF/CR. These property values differ from the values supported for the serial port object.

You can display the default communication property values for the VISA-serial object vs created in Creating a VISA-Serial Object with the get function.

get(vs,{'BaudRate','DataBits','Parity','StopBits','Terminator'})
ans = 
    [9600]    [8]    'none'    [1]    'LF'

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