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Appendix B Serial Port Information

Serial Communication Issues

This section explains some serial communication issues, including 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 both sends and receives 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, the devices

do not switch between transmitting and receiving.

In a differential serial bus (such as RS-422 or RS-485), a half-duplex

system transmits and receives 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 transmitter to all of the slave receivers. A second

transmission line connects all of the slave transmitters 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

shows a typical full-duplex system.

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National Instruments AT User Manual Page 134