TTi 121 series waveform generator, Labview error 1073807346: The given session or object reference is invalid.

Hi all, 

I’m trying to control a TTi TGA12102 waveform generator through labview. I’ve installed the appropriate drivers (Fluke x29 series) and connected the device with a serial to USB cable to the PC. The device shows up as ASRL3::INSTR “COM3” within NI MAX.

When using the example ‘Standard Waveform’ VI (installed with the driver software), I get an error at the VISA write block stating the session or object reference is invalid. 

I am a complete labview novice so any advice on what could be causing this error would be very much appreciated. 

Cheers.