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4.9
SCSI INTERFACE DESCRIPTION
This section briefly describes the SCSI protocol
implemented by
the Adaptec ACB-5500 controller.
The SCSI protocols are
described in detail in the ANSI X3T9.2 Small Computer System
Interface Specification, version 14.
4.1
GENERAL DESCRIPTION
OF SCSI
This system interface provides an efficient method of communica-
tion between a maximum of 8 computers and peripheral I/O devices.
The eight-port daisy-chained bus defined by the SCSI specifica-
tion supports the following features:
o
Single or multiple host system
o
Multiple peripheral devices and device types
o
Multiple overlap of peripheral device operations
o
Bus contention resolution through arbitration on a prioritized
basis
o
Asynchronous data transfer at up to 1.5 MBytes/sec.
o
Host-to-host communication.
Communication on the bus is allowed between two bus ports at a
time.
A maximum of eight
(8)
bus ports are allowed.
Each port
is attached to a n SCSI device (e.g.,
controller or host adapter) •
. When two devices communicate with each other on the bus, one acts
as an Initiator and the other acts as a Target.
The Target
(typically a controller) executes the operation.
A device will
usually have a fixed role as an Initiator or Target, but some
devices may be able to assume either role.
The ACB-5500 always
assumes a Target role.
An Initiator, or attached host adapter, may address up to four
logical units on an ACB-5500.
Each logical unit is a separate ST
506 disk drive having the characteristics (i.e., capacity) with
which it was formatted.
The ACB-5500 manages them as independent
un its,
k e e pin g a l l
nee e s s ar y pro g res s i n for mat ion
for
e a c h
device.
500504-00
4-1
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