Chapter 2 Installation and Verification
Serial for Windows NT
2-12
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National Instruments Corporation
Connecting Two-Wire Devices
The AT-485 boards and PCMCIA-485 cards are designed to work with
either two- or four-wire devices. If you are using a two-wire device, refer
to the device’s 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
AT-485 using half-duplex communication, you need a single twisted pair
of wires. At the AT-485, you should 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. You should connect the other end of
this wire to both the TXD+ and RXD+ signals on the data acquisition
device. You 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.
Refer to Chapter 4, Using Your Serial Hardware, for information on
setting the transceiver mode for two-wire communication. Refer to
Appendix B, Serial Port Information, for more information on duplex
architectures.
