Appendix B Serial Port Information
©
National Instruments Corporation
B-3
Serial for Windows NT
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.
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 B-1 shows a typical full-duplex system.
