National Instruments ENET-232 Series User Manual page 81

Termination
Tx
Rx
© National Instruments Corporation
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 to 120 Ω).
You can 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 D-3 shows a multidrop network using terminating resistors.
Slave 1
Tx
Rx
100 Ω
Figure D-3. Multidrop Network Using Terminating Resistors
Appendix D
Slave 2
Tx
Rx
D-5 ENET-232 and ENET-485 Series User Manual
Serial Port Information
Slave n
Tx
Rx
100 Ω