Exabyte VXA-2, Руководство по эксплуатации продукта, Страница: 47 / 90

D
ECEMBER
2003 VXA-2
37
6
O
VERVIEW OF THE
C
OMMAND
P
ROTOCOL
This chapter provides an overview of the command protocol used by the SCSI,
IDE/ATAPI, and FireWire models of the tape drive. It includes the following
topics:
4
Communication interface versus command protocol
4
SCSI-2 command protocol
C
OMMUNICATION
I
NTERFACE VERSUS
C
OMMAND
P
ROTOCOL
When a device is connected to a host computer, their interaction is
accomplished via a communication interface (for example, a parallel SCSI,
IDE/ATAPI, or FireWire bus). The communication interface is comprised of the
physical interface (for example, cables, connectors, and control circuitry) and
the signaling protocol used during communication.
The physical interface determines the number of devices that can be attached
to a bus or network loop, the maximum length of the cables, and the physical
characteristics of the cable itself (for example, the number of wires, shielding,
and so forth). The signaling protocol defines the electrical characteristics and
timing of signals carried by the cable, the message system requirements,
transmission speeds and maximum data transfer rates, as well as the encoding
and decoding of the individual bit patterns representing commands passing
between the individual devices.
The format and content of the information carried over the communication
interface, as well as how each device uses and responds to the information, is
governed by a command protocol . The command protocol determines how the
host (or initiator) interacts with the target device (for example, the tape drive)
by issuing commands to control its operation, transferring data, and
responding to status information. The target device responds to commands
from the host by performing the requested operation (for example, writing or
reading data on magnetic tape) and returning status information.