Device messages are transferred via the data lines of the GPIB bus, the "ATN" control line not being active. The ASCII character set is used. A distinction is made according to the direction in which device messages are transferred:
| Commands | are messages the controller sends to the instrument. They operate the device functions and request information. The commands are subdivided according to two criteria: |
| Setting commands | cause instrument settings such as a reset of the instrument or setting the output level to some value. | |
| Queries | cause data to be provided for output on the GPIB bus, e.g. for identification of the device or polling the active input. |
According to their definition in standard IEEE 488.2:
| Common Commands | have a function and notation that is exactly defined in standard IEEE 488.2. Typical tasks are the management of the standardized status registers, reset and selftest. | |
| Device-specific Commands | are functions that depend on the features of the instrument such as frequency setting. A majority of these commands has also been standardized by the SCPI committee (SCPI Introduction). | |
| Device responses | are messages the instrument sends to the controller after a query. They can contain measurement results, instrument settings and information on the instrument status. |
Structure and syntax of the device messages are described in the next section. In chapter Remote Control - Commands all commands are listed and explained in detail.