Specifications

Appendix C - Glossary of Terms RS-422 And RS-485 Applications Ebook
Page 109 of 137 Manual Documentation Number: <DocNumber>
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Four-wire (RS-422/RS-485)
A full duplex system uses one twisted pair (TDA(-) & TDB(+) for transmitting,
a second twisted pair (RDA & RDB) for receiving, plus a Signal
Ground/Common.
In a RS-422 system, there are normally two units (master-master or master-
slave), or one master with multiple listeners which do not transmit back.
In a Four-wire RS-485 system, there can be one or more masters, talking to
multiple slaves. Each slave must be addressable and have RS-485 transmitter
enable (as in Two-wire). A single master multi-slave system is sometimes
called "half-422" because the master is set for RS-422 operation, but the
slaves must be set for 485 transmit.
Full Duplex
A full duplex communications system is able to transmit and receive at the
same time. Two communications paths are provided, one for each direction.
Examples are RS-232, RS-422 and Four-wire RS-485. (Two-wire RS-485 is half
duplex)
G
Ground Loops
When two or more devices are connected to a common ground through
different paths, a ground loop occurs. Currents flow through these multiple
paths and develop voltages which can cause damage, noise or 50Hz/60Hz hum
in audio or video equipment. To prevent ground loops, all signal grounds need
to go to one common point and when two grounding points cannot be avoided,
one side must isolate the signal and grounds from the other.
Grounding Strap
A heavy solid or braided wire cable used to provide a low resistance path
from a device to a earth ground. Used with surge suppressors.