I have 4 34401a's setup up over Serial->USB connections (adapter+null modem cable) connected to my laptop running IO Libraries 17.3.whatever. Connection Expert sees the units, but they won't respond to a *IDN? query. I just get a VI_NSUP_OPER: Operation not supported Visa ErrorCode: 0xBFFF0067 (-1073807257). I've set the Interface to RS-232, the baud to 9600, and parity to none. GPIB is at 04 (not 31). Language to SCPI. The same settings are present in Connection Expert and in Windows Device Manager. I could swear I used the same computer to successfully connect to 1 unit and talk to it a week ago. It's got to be something simple I am missing, but I have no idea what it is. Can anyone help?