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AMM-1570/1572 User’s Guide
Stock Number: 510526-00, Rev. B Page: Glossary-4
Print Spec Number: 492379-00
Current Date: 3/7/94 ECN Date: 3/8/94
AMM-1570/1572 User’s Guide
Glossary-4
Bus
A pathway for data in a com-
puter system. All PCs have an
expansion bus, which is
designed to host add-on (expan-
sion) devices, such as modems,
adapter boards, and video
adapters. Expansion devices use
the bus to send data to and
receive data from the PC's CPU
or memory. ISA, EISA, Micro
Channel
®
, and VL-Bus™ are the
major bus standards used in PCs.
Bus Device Reset
A SCSI message that clears all
pending activity in the SCSI
peripheral device target to
which it is addressed.
Bus Mastering
A high performance method of
data transfer in which the host
adapter's onboard processor
handles the transfer of data
directly to and from a comput-
er's memory without interven-
tion from the computer's
microprocessor. This is the fast-
est method of data transfer
available for multitasking oper-
ating systems.
Byte
An 8-bit unit of data. A byte is
normally the smallest address-
able unit of memory and the
smallest unit of transfer on the
SCSI bus.
Device Driver
A software program that enables
a PC to communicate with
peripheral devices, such as fixed
disk drives and CD-ROM drives.
Each kind of device requires a
different driver. Device driver
programs are stored on a PC's
fixed disk and are loaded into
memory at boot time.
Differential
A term referring to the electrical
characteristics of the signals used
on the SCSI bus interface. Differ-
ential signals occupy two con-
ductors with a positive (+) and
negative (-) polarity component
of the signal. This minimizes the
effect of common mode signal
noise and allows the SCSI bus to
operate reliably over greater
distances at a higher speed.
Direct Memory Access
A mechanism that allows hard-
ware control of the transfer of
streams of data to or from the
main memory of a computing
system. The mechanism may
require setup by the host soft-
ware. After initialization, it auto-
matically sequences the required
data transfer and provides the
necessary address information.
Disconnect/Reconnect
Disconnect is the function that
occurs when a target releases
control of the SCSI bus, allowing
the bus to go to the Bus Free
phase. Reconnect is the function
that occurs when a target selects
an initiator to continue an opera-
tion after a disconnect.