ATTO Technology UL4D PCI Installation And Operation Manual page 7

Term Definition
double transition Increases the data line frequency to equal that of the request signal, allowing sampling on
clocking both the leading and trailing edges of the request signal. Clocking can be set to ensure
compatibility with legacy devices.
flow control The target indicates to the initiator when the last packet of a data stream will be transferred so
that the initiator can flush FIFOs and terminate pre-fetch sooner than previously possible.
Basically, the target warns the initiator that the transfer is almost complete so that it can
prepare for the next transfer while the target completes the current transfer.
host A processor, usually a CPU and memory, which communicates with devices over an interface
HVD High voltage differential: uses two wires, transmitting a signal on one and its inverse on the
other. At the receiving end, the difference between the two signals is measured and
interpreted. Noise on the bus will affect both the signal and its inverse equally, so the
difference between the two lines will remain the same and the noise cannot be misread as a
signal.
initiator device A component which originates a command
LED Light-emitting diode: a type of diode that emits light when current passes through it. Visible
LEDs are used as indicator lights on all sorts of electronic devices.
LVD Low voltage differential. SCSI signalling method that combines the benefits of HVD and
single-ended technologies, allowing longer cabling configurations while consuming less
power than HVD technology.
originator An initiating device; a component which originates a command
packetization Creates information units (IUs) from commands, data, status information, etc. which are
passed as synchronous transfers. Maximizes bus use, minimizes command overhead and
allows multiple commands to be transferred in a single connection
pre-compensation Although SCSI transfer speeds have changed dramatically over the past several generations,
cable specifications have remained constant. Higher speed and frequency signals have a
greater potential for reflection and distortion over distance. Pre-compensation techniques
slightly modify the SCSI signal to reduce the chance of these types of problems.
Quick Arbitration The process of devices negotiating for control of the bus with built-in "quiet times" so that fast
Select (QAS) and legacy devices have an opportunity to take control of the bus. A fair, but somewhat
Arbitration inefficient process, QAS speeds up the arbitration process by eliminating the bus free phase.
When combined with packetization, reduces command overhead and maximizes bus use.
read and write data Minimizes data transfer overhead by allowing a target to send one data stream (LQ) packet
streaming followed by multiple data packets. Minimizes overhead of data transfers because the target
can send one data stream packet followed by multiple data packets
receiver The ultimate destination of data transmission; a terminal device
SCSI Small Computer Systems Interface: a processor-independent standard for system-level
interface between a computer and intelligent devices including hard disks, floppy disks, CD-
ROM, printers, scanners, etc.
single-ended An electrical signal protocol that transmits information through changes in voltage. Single-
ended SCSI uses standard TTL signal and ground pairs to transmit information over the SCSI
bus.
training pattern SCSI is a parallel bus technology that is dependent on signals being transmitted on parallel
wires simultaneously. At higher speeds, minute differences in wire lengths and transmission
characteristics could cause problems. Training pattern testing measures these minute
differences and compensates for them.
Vpath™ Technology The ATTO ExpressPCI UL4S with Vpath Technology offers data transfer rates of 320
MB/sec. With one external connector and one internal connector, Vpath Technology allows
both faster and slower devices to run without impacting the speed of faster devices.
SCSI is a key technology for storage
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