Appendix B Serial Port Information
©
National Instruments Corporation
B-5
Serial for Windows NT
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 connector that contains embedded terminating
resistors for easy termination.
Figure B-3 shows a multidrop network using terminating resistors.
MASTER
Slave 1
100
Ω
Slave 2
Slave n
100
Ω
Tx
Tx
Rx
Tx
Rx
Tx
Rx
Rx
Figure B-3.
Multidrop Network Using Terminating Resistors
Bias Resistors
A transmission line enters an indeterminate state if no nodes are
transmitting on it. This indeterminate state can cause the receivers to
receive invalid data bits from noise picked up on the cable. To prevent
these data bits, you should force the transmission line into a known state.
By installing two 620
Ω
bias resistors at one node on the transmission
line, you can create a voltage divider that forces the voltage between the
differential pair to be less than 200 mV, the threshold voltage for the
receiver. You should install these resistors on only one node.
