User's Guide

SCSI Interface Description Page 7
Data-transfer operations on the SCSI bus are either asynchronous or synchronous.
Asynchronous data transfer operations follow a defined request/acknowledge
(REQ/ACK) handshake protocol. Two eight-bit bytes of information can be
transferred with each REQ/ACK handshake. The default data transfer mode is
asynchronous.
Synchronous data transfer operations are initiated through the SYNCHRONOUS
DATA TRANSFER REQUEST message from the Initiator.
The SCSI bus protocol is divided into three modes or phases of operation: Waiting
Phases, Control Phases, and Information Transfer Phases. These phases are
further subdivided into the eight operational phases as listed in the following table.
Information on these operational phases is contained in the following paragraphs.
Waiting Phases Control Phases Information Phases
1. Bus Free 2. Arbitration 5. Command
3. Selection 6. Data (Data In/Data Out)
4. Reselection 7. Status
8. Message (Message In/
Message Out)
Waiting and Control Phases
The status of the SCSI bus is a function of the control signals. These signals define
the SCSI as in the Waiting Phase (Bus Free Phase), the Control Phases
(Arbitration, Selection, or Reselection), or the Information Transfer Phases
(Command, Data, Status, or Message). Seagate DDS-4 and DAT 72 drives support
both a SCSI system with Arbitration Phase and a SCSI system without Arbitration
Phase (non-arbitrating system).
Arbitrating Systems
In arbitration systems (required in SCSI-2), the sequence of SCSI bus phases
follows the sequence shown in Figure 2-1 on the following page. Operation begins
with the Bus Free Phase. Normal progression is from the Bus Free to the Arbitration
Phase. During Arbitration, Initiators and Targets assert for control of the SCSI bus.
The bus is awarded to the device with the highest priority SCSI bus address.