You have to assert the REN line and address the device to listen. If
you are using NI-488.2, this should be done automatically by the driver
if you are using device-level calls. If you are using board-level
commands (or IEEE 488.2-style commands), you would use either ibsre (or
EnableRemote). The addressing of the device to listen will happen
automatically when you send the first string to the device.
Good luck.