Table 4-4. Operational Phases - Viper 60s Theory Of Operation And Maintenance Manual

INTERFACE
4.5 SCSI Bus Protocol
Communication on the SCSI bus occurs between a host computer SCSI controller and a
peripheral controller. The host controller is the Initiator and the peripheral device is the
Target.
Some SCSI bus functions are assigned to the Initiator and others to the Target. The
Initiator arbitrates (enters the Arbitration Phase of operation) for control of the SCSI bus
and enters the Selection Phase to select a specific Target such as a Viper drive.
The Target drive can request transfer of command, data, status, information, or discon-
nect from the bus. While the Viper drive is disconnected, the bus is free to accomplish
other tasks. While disconnected, the Target drive can process information obtained from
the bus. If the Target wants to reconnect,
it
arbitrates for bus control. For example, a
Target can res elect an Initiator or another Target to continue an operation.
The Target at times actually becomes an Initiator and arbitrates for control of the SCSI bus.
For the Viper drive this occurs only during a Copy operation.
SCSI bus data transfer operations are asynchronous as defined in the ANSI SCSI specifica-
tion and follow a defined request/acknowledge (REQI ACK) handshake protocol. One
eight-bit byte of information can be transferred with each REQ/ ACK handshake.
The SCSI bus protocol is divided into three modes of operation: Waiting Phases, Control
Phases, and Information Transfer Phases. These are subdivided into the eight operational
phases listed in Table 4-4. Information on these operational phases is contained in the
following paragraphs.
F
WAITING
PHASES
Table 4-4. Operational Phases
. -
-O-PERA~TI6N~AL~P-H~ASES-~~-·······--~
1
CONTROL
PHASES
INFORMATION
TRANSFER
J
L 1 Bus free 2. Arbitration 5. Command
. 3. Selection 6. Data (Data In/Data Out)
_ _ ___ 4. Res_e_lec_ti_on_ . . __
~:_~_t:_!~;g_e (Me=sa_g_e_ln_/M_e_ss~g_e_ou_t)~~
4-5