Setting up a GPIB communication with Keithley 220 and HP 34401A

Hi everyone,

I am using GPIB-USB-HS to connect both Keithley 220 Current Source and HP 34401A Multimeter and send commands via LabVIEW. I am using an existing old LabVIEW code, but I keep running into the problem (Error 7, file not found) when I send the command through GPIB Write Function, "INP:IMP:AUTO ON" (which sets the multimeter in high impedance).

And so, I am climbing down to the low level subVIs. However, it looks like I can't even run this simple voltage reader VI. There is simply no output string when I run the program, and so I assume I am not communicating with 34401A

In order to figure out whether the devices are connected to the computer via usb cable, I ran NI MAX program, and I GPIB-USB-HS "GPIB1" is visible under Devices and Interfaces.

What am I missing? One of my confusions is how to set GPIB address of two devices. Do I set them with the same address? I am confused then how, when sending a GPIB Write command, we can select one of the two devices (220 and 34401A).