User Manual

42 MHX-2400 Operating Manual: Appendix H . Glossary
can be controlled also by software using X-ON/X-
OFF (transmitter on/off) commands.
Frequency-hopping A type of spread spectrum
communication whereby the carrier frequency
used between transmitter and receiver changes
repeatedly in a synchronized fashion according to
a specified algorithm or table. This minimizes
unauthorized jamming (interference) and
interception of telecommunications.
Full-duplex Where data can be transmitted,
simultaneously and independently, bi-
directionally.
Half duplex Exists when the communications
medium supports bi-directional transmission, but
data can only travel in one direction at the same
time.
Handshaking A flow-control procedure for
establishing data communications whereby
devices indicate that data is to be sent and await
appropriate signals that allow them to proceed.
Line-of-sight Condition in which a transmitted
signal can reach its destination by travelling a
straight path, without being absorbed and/or
bounced by objects in its path.
Master The station which controls and/or polls one
or more Slave stations in a point-to-point or point-
to-multipoint network. Often functions as a server
or hub for the network.
Non-volatile memory Memory which retains
information which is written to it.
Null modem cable See Crossover cable.
Point-to-point A simple communications network
in which only two DTEs are participants.
Point-to-multipoint A communications network
in which a Master DTE communicates with two or
more Slave DTEs.
Repeater A device which automatically amplifies
or restores signals to compensate for distortion
and/or attenuation prior to retransmission. A
repeater is typically used to extend the distance
for which data can be reliably transmitted using a
particular medium or communications device.
RS-232 (Recommended Standard 232; more
accurately, RS-232C or EIA/TIA-232E) Defined
by the EIA, a widely known standard electrical
and physical interface for linking DCEs and DTEs
for serial data communications. Traditionally
specifies a 25-pin D-sub connector, although
many newer devices use a compact 9-pin
connector with only the essential signaling lines
used in asynchronous serial communications.
Lines have two possible states: “high” (on, active,
asserted, carrying +3 to +25 V) or “low” (off,
inactive, disasserted, carrying -3 to -25 V).
RTU (Remote Terminal Unit) A common term
describing a DTE device which is part of a wide-
area network. Often a RTU performs data I/O and
transmits the data to a centralized station.
Serial communications A common mode of
data transmission whereby character bits are sent
sequentially, one at a time, using the same
signaling line. Contrast with parallel
communications where all bits of a byte are
transmitted at once, usually requiring a signal line
for each bit.
Shielded cable Interface medium which is
internally shrouded by a protective sheath to
minimize external electromagnetic interference
(“noise”).
Slave A station which is controlled and/or polled by
the Master station for communications. Typically
represents one end of a point-to-point connection,
or one of the terminal nodes in a point-to-
multipoint network. Often a RTU is linked by a
Slave DCE.
Spread spectrum A method of transmitting a
signal over a wider bandwidth (using several
frequencies) than the minimum necessary for the
originally narrowband signal. A number of
techniques are used to achieve spread spectrum
telecommunications, including frequency hopping.
Spread spectrum provides the possibility of
sharing the same band amongst many users while
increasing the tolerance to interference and noise,
and enhancing privacy of communications.
Throughput A measure of the rate of data trans-
mission passing through a data communication
system, often expressed as bits or characters per
second (bps or cps).