62 IP920 Operating Manual: Appendix F 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 Remote 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 Remote 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).
RS-422 (Recommended Standard 422; more
accurately, EIA/TIA-422) Defined by the EIA, a
widely known standard electrical and physical
interface for linking DCEs and DTEs for serial
data communications. This standard specifies a
single, unidirectional driver with multiple
receivers.
RS-485 (Recommended Standard 485; more
accurately, EIA/TIA-485) Defined by the EIA, a
widely known standard electrical and physical
interface for linking DCEs and DTEs for serial
data communications. This standard specifies
bidirectional, half-duplex data transmission, is the
only EIA/TIA standard that allows multiple
receivers and drivers in "bus" configurations. RS-
485 parts are backward-compatible and
interchangeable with their RS-422 counterparts,
but RS-422 drivers should not be used in an RS-
485 system because they cannot relinquish control
of the bus.
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”).
Remote 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 Remote 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
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