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Appendix B Serial Port Information
PCMCIA Serial for Windows 98/95 B-4 © National Instruments Corporation
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.
Figure B-1. Typical Full-Duplex System
Half Duplex
A typical half-duplex multidrop bus architecture also involves a
master-slave protocol. In a half-duplex system, all transmitters and
receivers are connected to the same transmission line. A half-duplex system
is often referred to as a two-wire system. Figure B-2 shows a typical
half-duplex system.
Figure B-2. Typical Half-Duplex System
MASTER
Slave 1 Slave 2 Slave n
Tx
Tx
Rx
Tx
Rx
Tx
Rx
Rx
MASTER
Slave 1 Slave 2 Slave n
Tx
Tx
Rx Rx
Tx
Rx
Tx
Rx