The ``SCSI bus'' transfers data and state information between interconnected SCSI devices. A single transaction between an ``initiator'' and a ``target'' can involve up to 8 distinct ``phases.'' These phases are almost entirely determined by the target (e.g., the hard disk drive). The current phase can be determined from an examination of five SCSI bus signals, as shown in Table [LXT91, p. 57,].
Table: SCSI Bus Phase Determination
Some controllers (notably the inexpensive Seagate controller) require direct manipulation of the SCSI bus--other controllers automatically handle these low-level details. Each of the eight phases will be described in detail.