call Device Application or use VDT2?

You might be able to call Communicator using ExecuteScriptText if Communicator can be driven by batch commands, e.g., from a DOS window.

A better approach would be to use VDT2 to send the commands to the device yourself. Start by reading the documentation for the device to understand what commands it requires. Make note of whether it requires CR, LF or CRLF to terminate a command.

Then start reading the "VDT2 Help" file (in "Igor Pro Folder\More Extensions\Data Acquisition").

All you need to do is to set the port up:

VDT2 /P=COM1 baud=9600, databits=7, stopbits=2, . . .

and then send a command:

VDTOperationsPort2 COM1
VDTWrite2 "Measure\r\n"  // \r is CR, \n is LF

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I vote for the RS232 option. Communicating with the Keithley device is very easy. I'm using the same model, although use GPIB for communications.
First, you can try this:
1) set up the 6514 for RS 232 communication (see manual chapter 12 'Remote Operation')
2) load the VDT2 XOP in Igor
3) go to Misc -> VDT2 -> VDT2 settings and configure the com port to match you requirements
4) open the VDT2 Window from the same menu and communicate with the instrument by actually typing in the commands (make sure the 'terminal port' is set to the right com port)

With this you can just try out, how to get the results you want and set up the device. After this you can build your procedure with the commands. For example:

Function Keithley_Init()
    VDTWrite2 "*RST"             // resets Keithley to default conditions
    VDTWrite2 "trace:clear"          // clear data buffer
    VDTWrite2 "configure:current"        // configure for 'current' measurement
    VDTWrite2 "format:elements reading"  // configure to just give the value reading (no timestamp etc.)
End

Expand this as you wish... (see chapter 15 'SCPI Signal Oriented Measurement Commands' for all commands). If you are not using the default settings for the com port, I suggest you add a line at the top to configure this every time (see post from hrodstein).

To read the data then use something like this:

Function Keithley_ReadData()
    String Response
    VDTWrite2/Q/O=1 "read?"  // request readout (flag demonstration: /Q to suppress errors and /O=1 for 1 second timeout)
    VDTRead2/T="" Response   // receive the value string

    return str2num(Response) // convert to variable and return (only works, if just value is sent and nothing else...
                 // ... otherwise you have to extract the value from the response string.)
End

RS232 has the problem that a connection loss or some other errors in the device are not detected. Use timeouts and some error handling for this to prevent 'hanging' of you procedure (use ctrl-break to abort the command manually if this occurs).

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