SECTION 4
SCSI INTERFACE
4.1
INTRODUCTION
The
ProDrive 40S and
ProDrive
80S disk drives are "intelligent" disk drives in that they feature
an
integrated Small Computer System Interface (SCSI) controller. This section dermes the functional
requirements
for
interfacing with the
ProDrive
4OS/80S via the SCSI bus, including the command set,
bus timing, and other parameters. The paragraphs below are intended
to
provide a brief description
of
this
interface. Refer to
Section 5 for more detailed infonnation on the
ProDrive
4OS/80S programming
requirements.
4.2 ANSI CONFORMANCE
4.2.1
Confonnance
Level and Altematlves
The
ProDrive 40S and 80S
confonn
to
ANSI X3T9.2/82-2 Revision 17B at confonnance level 2 with the
following alternatives supported: single-ended drivers, tenninator power, parity,
hard
reset, and reservation
queuing. In addition, the synchronous data ttansfer option is implemented with a minimum
ttansfer period
of
250 nanoseconds and a maximum
REQI
ACK offset
of
8.
4.2.2
Hardware
Imple~ntation
Choices
In many cases, the ANSI specification allows more than one option. When more than one option is
allowed, and only one option was implemented, the decision as to which option
to
implement was based on
the following:
4.3
4.3.1
• . All SCSI bus drivers are OR-tied type drivers and do not actively drive any signals to the false
state.
• During arbitration,
if
the
ProDrive
loses arbitration due to a higher priority SCSI
ID
bit
being driven
on
the SCSI data bus, it will release its signals without waiting for SEL to
become true.
• During arbitration phase, DB(P) will
be
undriven.
SCSI INTERFACE CHARACTERISTICS
GENERAL DESCRIPTION
The
SCSI bus can support up
to
eight SCSI· devices in any combination
of
initiators and targets.
The
ProDrive
always assume a fixed role as a target.
Certain
SCSI bus functions are assigned
to
the initiator
or
the target:
•
The
initiator may arbitrate for the SCSI bus and select a particular target.
•
..
11le target may request the transfer
of
COMMAND, DATA, STATUS,
or
other infonnation on
the DATA
BUS, and in some cases
it
may arbitrate for the SCSI bus and reselect an initiator
for the purpose
of
continuing an operation.
Communication with the
ProDrive
takes place
on
the SCSI
OAT
A
BUS
in asynchronous
or
synchronous
mode, using a dermed REQ/ACK handshake protocol.
SCSllnteriace 4 • 1
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