We recently purchased a used HP8720C Firmware REV 01.05 Oct 15,1992. We have an HP8722ES that has been working fine via an HP82357A USB to GPIB controller. When trying to control the 8720C in the same manner as the 8722ES it shows up in the list of instruments on the GPIB at address 22 while the 8722ES is at address 16. I am not however able to connect to the 8720C. There is the 8722ES listed with Network Analyzer next to the 8722ES and the 8720C is listed with no name next to 8720C. When selected the connect button is selectable with the 8722ES. When the 8720C is selected it is not selectable with the connect button. They both use the same instrument drivers and the Agilent I/O control is version 14.2.8931.1 VISA. Any help would be appreciated. Thanks in advance.