AppendixB SerialPort Information
©NationalInstruments Corporation B-5 SerialHardware and Software for Windows
Termination
Because eachdifferential pair of wires is a transmission line, you must
properly terminate the line to preventreflections. A common method of
terminatinga t wo-wire multidrop RS-485 network is to install terminating
resistors at eachend of the multidrop network. If you daisy-chained
multiple instruments together,you need a terminating resisto ra t onlyt he
firstand last instruments. The termin ating resistor should match the
characteristicimped ance of the transmission line (typically 100 to 120 Ω).
Youcan order an optional DB-9 RS-485 termination connector that
contains embedded terminating resistors for easy termination from
National Instruments. For ordering information, contact National
Instruments.
Figure B-3 showsa multidrop network using terminating resistors.
FigureB-3. MultidropNetwork Usi ng TerminatingResistors
Bias Resistors
A transmission line enters an indeterminate state if no nodes are
transmitting on it.This indeterminatestate can cause the receivers to
receiveinvalid data bits from noise picked up on the cable. Toprevent a line
fromreceiving these data bits, force the transmission line into a known
state.To do so, install two 620 Ωbias resistors at one node on the
transmissionline; doing so creates a voltagedividerthat forces the voltage
between the differentialpair to be greater than 200 mV, the threshold
voltagefor the receiver. Youshould install these resistors on only one node.
Note Bias resistorsare n otavailable on the eight-port PCI-485 or eight-port PXI-8421.
MASTER
Slave1
100Ω
Slave2 Slaven
100Ω
Tx
Tx Rx Tx Rx Tx Rx
Rx