AppendixB SerialPort Information
©NationalInstruments Corporation B-3 SerialHardware and Software for Windows
Serial Communication Issues
Thissection explains some serial co mmunication issues, including duplex
architectures,termination methods, bias resistors, and types of connecting
equipment.

Duplex Architectures

Duplexrefers to the means of bandwidth usage in a serial system. The two
commonmeans of bi-directi onal serial communication are full duplex and
half duplex.Half-du plexcommuni cation involvesa transmitter and a
receiverconnected to each end of the same wire or pair of wires. Because
the same transmission line both sends and receivesdata, devices cannot
send data in both directions at the same time. First, one devicetr ansmits
overthe wire(s) to the receiver of the second device. When the first device
finishestransmitti ng, both devicesswitch the connections from their
transmittert o their receiver,or vice versa. The device that was receiving
data can then transmit overthe line.
In full-duplex communication, the devices use a separate wire (or pair of
wires)for simultaneous transmissionin each direction. Thus, the devices
do notsw itch between transmitting and receiving.
In a differentialserial bus (such as RS-422 or RS-485), a half-duplex
system transmits and receivesover the same twisted pair of wires.
Thus,h alf-duplexcommunication is often referred to as two-wire
communications.Likewise, full-duplex communication is often referred
to as four-wire communications,because the f ull-duplexsystem uses a
separatepair of wires for communication in each direction.

Full Duplex

A typical full-duplex multidrop bus architecture involvesa master-slave
protocol.Only one device, the master, can control access to the bus. All
otherdevices are slaves. Slave devicesmust wait for the master to give them
accessto the bus. In a typical full-duplex system, one transmission line
connectst he busmaster transmitter to all of the slave receivers. A second
transmission line connects all of the slavetransmitters to the bus master
receiver.Because each transmission line has two separate wires, a
full-duplex system is often referred to as a four-wire system. Figure B-1
showsa typical full-dup lexsystem .