Appendix C
Serial Port Information
PCI Serial for Windows 95
C-4
©
National Instruments Corporation
Figure C-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 C-2 shows a typical
half-duplex system.
Figure C-2. Typical Half-Duplex System
Termination
Because each differential pair of wires is a transmission line, you must
properly terminate the line to prevent reflections. A common method of
terminating a two-wire multidrop RS-485 network is to install terminating
resistors at each end of the multidrop network. If you daisy-chained
multiple instruments together, you need a terminating resistor at only the
first and last instruments. The terminating resistor should match the
characteristic impedance of the transmission line (typically 100 –120
Ω
).
National Instruments offers an optional DB-9 RS-485 termination
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