National Instruments ENET-232 Series, Manual

Page: 135 / 237

Connecting Two-Wire Devices
The NI RS-485 hardware is designed to work with either two or four-wire
devices. If you are using a two-wire device, refer to the device
documentation for specific wiring instructions.
In general, half-duplex networks use a single twisted pair of wires for
communication in both directions, so you must connect both the
transmitter and the receiver at each end of the same pair of wires. For
example, to connect an RS-485 data acquisition device to a port on your
board using half-duplex communication, you need a single twisted pair of
wires. At the board, connect the TXD+ and RXD+ signals (pins 8 and 4
on a DB-9 connector, pins 5 and 20 on a DB-25 connector) together and
to one wire. Connect the other end of this wire to both the TXD+ and
RXD+ signals on the data acquisition device. Use the same method to
connect the TXD– and RXD– signals (pins 9 and 5 on a DB-9 connector,
pins 22 and 7 on a DB-25 connector) to the second wire.
For information about setting the transceiver mode for two-wire
communication, refer to
RS-485 Transceiver Control . For more
information about duplex architectures, refer to Serial Port Information .