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Appendix B Serial Port Information
© National Instruments Corporation B-3 PCMCIA Serial for Windows 98/95
Serial Communication Issues
This section explains some serial communication issues, such as duplex
architectures, termination methods, bias resistors, and types of connecting
equipment.
Duplex Architectures
Duplex refers to the means of bandwidth usage in a serial system. The two
common means of bi-directional serial communication are full duplex and
half duplex. Half-duplex communication involves a transmitter and a
receiver connected to each end of the same wire or pair of wires. Because
the same transmission line is used for both sending and receiving data,
devices cannot send data in both directions at the same time. First, one
device transmits over the wire(s) to the receiver of the second device. When
the first device finishes transmitting, both devices switch the connections
from their transmitter to their receiver or vice versa. The device that was
receiving data can then transmit over the line.
In full-duplex communication, the devices use a separate wire (or pair of
wires) for simultaneous transmission in each direction. Thus, there is no
need to switch between transmitting and receiving.
In a differential serial bus (for example, RS-422 or RS-485), a half-duplex
system can transmit and receive over the same twisted pair of wires.
Thus, half-duplex communication is often referred to as
two-wire
communications. Likewise, full-duplex communication is often referred
to as
four-wire communications, because the full-duplex system uses a
separate pair of wires for communication in each direction.