Filter
Top Products
A-61124 March 1999 5-1
5 The SCSI Interface
This chapter describes the SCSI interface used with the scanners.
For complete information on the appropriate SCSI specification,
refer to SCSI-2 Working Draft ANSI X. 131-198X, Revision 6,
10/29/88.
SCSI overview
The SCSI interface provides a means of communication between
a maximum of eight computer and peripheral devices, giving the
host computer independence within this system. As a result, tape
drives, printers, optical disks, communications devices, etc., can
be added to the host computer(s) without requiring modification to
the generic system hardware or software. The interface uses
"logical" rather than "physical" addressing for all data structures.
SCSI bus
The SCSI bus allows communication between any two SCSI
devices at a time. When two SCSI devices communicate on the
bus, one acts as an initiator and the other acts as a target. The
initiator is usually a host computer that originates operations, and
the target is usually a peripheral controller that performs the
operation. A SCSI device generally has a fixed role as an initiator
or target, but some may assume either role. The scanner acts
only as a SCSI target.
Access to the SCSI bus is handled through bus arbitration. The
SCSI device with the highest priority (as determined by its SCSI
ID bit) is given control of the bus. The SCSI device with an ID of 7
is the highest priority device. The initiator then selects a target
and the target controls all further communications. Data transfers
on the bus are asynchronous and follow a Request/Acknowledge
handshake protocol. One 8-bit byte of information is transferred
to the initiator with each handshake.
NOTE: The amount of time required to execute the SCSI
commands is affected by the number of peripheral devices
on the bus, as well as the priorities assigned to each
peripheral device. Therefore, the amount of time required
to execute the SCSI commands will vary based upon the
system configuration. It is recommended that a dedicated
host adapter be used with the scanner.