Viper 60s Theory Of Operation And Maintenance Manual page 37

VIPER SCSI 60, 125
&
150 THEORY OF OPERATION AND MAINTENANCE
4.6 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).
Viper drives support both a SCSI system with Arbitration Phase and a SCSI system
without Arbitration Phase.
4.6.1 Nonarbitrating Systems
In systems where the Arbitration Phase is not implemented, the allowable sequences are
shown in Figure 4-1. The normal progression
is
from the Bus Free Phase to the Selection
Phase and from the Selection Phase to one or more of the Information Transfer Phases. See
ANSI SCSI X1.31 for detailed discussions of bus timing.
BUS FREE
PHASE
RESET CONDITION
~".::
. .
:::::~~"""'*'~~~,
COMMAND, DATA, :,:
SELECTION
PHASE
I
;
I!<';..-_ .....
~
STATUS, OR
~ , MESSAGE
PHASES
Figure 4·1. Phase Sequencing with Nonarbltration
4.6.2 Arbitrating Systems
In arbitration systems, the sequence of SCSI bus phases follows the sequence shown in
Figure 4-2. 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 (DB7).
Arbitration is won by the highest priority SCSI device when both BSY and SEL are
asserted and a delay of at least 1200 nSec
(1
bus clear delay + 1 bus settle delay) occurs
before the Arbitration Phase ends and the Selection Phase begins. Signal timing
is
shown
in Figure 4-3 and complete Arbitration Phase protocol is described in Table 4-5.
4-6