Unlisten, Untalk, and addressing a device to talk or listen
To address a device to listen, you send it a command byte which is simply its GPIB address, plus 32 (i.e. the hex value 0x20, i.e. bit number 6). To address a device to talk, the command byte is its address plus 64 (i.e. the hex value 0x40, i.e. bit number 7).
Valid GPIB address are 0-30. (It seems that by convention, the controller uses address 21).
GPIB address 31 is used for the "untalk" and "unlisten" commands.
My Keithley 196 is currently configured to use address 17 (which is hex 0x11)
To address it to listen, I'd send the command byte 0x31 (17 + 32, or 0x11 + 0x20).
To address it to talk, I'd send the command byte 0x51 (17 + 64, or 0x11 + 0x40).
To command all devices to stop listening ("unlisten"), the command byte would be 0x3F (31 + 32, or 0x1F + 0x20).
To command all devices to stop talking ("untalk"), the command byte would be 0x5F (31 + 64, or 0x1F + 0x40).