This question (and your similar post in the discussion about JSON) doesn't really belong here in LabVIEW Robotics. You might want to post it here, instead...
http://forums.ni.com/t5/Instrument-Control-GPIB-Serial/bd-p/140
However, when I look at the programming protocol for that device, I have to say it is an especially unfriendly device for programmatically accessing the voltage readings.
Among the problems in your VI...
* The device speaks a fixed 2400 baud, and you are using 9600 baud.
* The device speaks 7 bits, odd parity, one stop bit, with DTR/DSR flow control, and you haven't configured any of those
* The device just spews data, so there's no need for you to write anything to it (I think)
* The device spews out the settings for the LEDs that make up the seven segment display on its front panel. You will have to convert those to actual numeric values, probably through a lookup table. Then plot those converted numeric values.
Do you have the opportunity to use a different multimeter? Several are much more easily programmed in LabVIEW. The RS-232 interface for the GDM-396 seems to have been designed to attach another display, not for capturing programmatic readings. You might look at http://ni.com/idnet and browse for the list of multimeters supported with existing LabVIEW drivers.
Also, have you read the "Controlling Instruments" section of the LabVIEW help? (It may have been called something different in LabVIEW 7.)
