Encom ENC-900, Operating Manual

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ENC-900 Operating Manual

: 

Appendix D Glossary

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, ca3 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”). 

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 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).