HP PCIe U320 SCSI Host Bus Adapter - Manual (Page 56)

is almost complete so that it can prepare for the next transfer while the target
completes the current transfer.
full duplex A communication protocol which allows transmission in both directions at the
same time.
half duplex A communication protocol which allows transmission in both directions, but
only one direction at a time.
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 many electronic devices.
LUN Logical Unit Number. An address for a component of a SCSI device, similar to
an apartment number. In a library, for example, the host computer might send
the SCSI commands for the library to LUN1 of the tape drive and send SCSI
commands for the tape drive itself to LUN 0.
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
parity checking A method that verifies the accuracy of data transmitted over the SCSI bus by
adding one bit in the transfer to make the sum of all the bits either odd or even
(for odd or even parity). An error message occurs if the sum is not correct.
PCI Peripheral Component Interconnect. Allows peripherals to be connected directly
to computer memory, bypassing the slower ISA and EISA busses.
point-to-point A topology where two ports communicate.
port An access point in a device.
port address The address, assigned by the PCI bus, through which commands are sent to a
host adapter board. Also called port number.
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
Select (QAS) Arbi-
tration
The process of devices negotiating for control of the bus with built-in “quiet
times” so that fast and legacy devices have an opportunity to take control of the
bus. A fair, but somewhat inefficient process, QAS speeds up the arbitration
56 Glossary