National Instruments PCMCIA-232 Getting Started page 33

Serial Communication Issues
Duplex Architectures
© National Instruments Corporation
This section explains some serial communication issues, such as duplex
architectures, termination methods, bias resistors, and types of connecting
equipment.
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.
Full Duplex
A typical full-duplex multidrop bus architecture involves a master-slave
protocol. Only one device, the master, can control access to the bus. All
other devices are slaves. Slave devices must wait for the master to give them
access to the bus. In a typical full-duplex system, one transmission line
connects the bus master's transmitter to all of the slave receivers. A second
transmission line connects all of the slave transmitters to the bus master's
receiver. Because in a differential system each transmission line is
composed of two separate wires, a full-duplex system is often referred to as
a four-wire system. Figure A-1 shows a typical full-duplex system.
Appendix A
A-3
Serial Port Information
PCMCIA Serial for Windows NT