Communication begins when the new AS-Interface master port configuration is downloaded and consists of three phases. In the first phase, the master polls each slave device and requests the slave's profile (I/O configuration and Identification code). This is a fixed-format description of the device that is referred to as the AS-Interface Device Type in the DeltaV software. In the second phase, the master sends a four bit parameter value to each slave. The parameter value controls specialized, non-standard functions of the device such as dark-on rather than the standard light-on for a photosensor. In the third phase, the master sends each slave its output values and receives each slave's input values.