ibln() checks for the presence of a device, by attempting to address it as a listener. ud specifies the GPIB interface board which should check for listeners. If ud is a device descriptor, then the device's access board is used.
The GPIB address to check is specified by the pad and sad arguments. pad specifies the primary address, 0 through 30 are valid values. sad gives the secondary address, and may be a value from 0x60 through 0x7e (96 through 126), or one of the constants NO_SAD or ALL_SAD. NO_SAD indicates that no secondary addressing is to be used, and ALL_SAD indicates that all secondary addresses should be checked.
If the board finds a listener at the specified GPIB address(es), then the variable specified by the pointer found_listener is set to a nonzero value. If no listener is found, the variable is set to zero.
The value of ibsta is returned.