Vaisala HMT330 - Manual (Page 61)

Chapter 3 ________________________________________________________________ Installation
VAISALA ________________________________________________________________________ 59
5. Connect the twisted pair wires (1 or 2 pairs) to the screw terminals
as presented in Table 6 below:
Table 6 Connecting the Twisted Pair Wires to the Screw
Terminals
Screw Terminal Data Line
(2-wire RS-485) Data Line
(4-wire RS-422/485)
1 (not connected) Rx D0-
2 (not connected) Rx D1+
3 Common Common
4 D0- Tx D0-
5 D1+ Tx D1+
To avoid problems in the RS-422/485 network, the Common
terminal should be wired to the corresponding terminals of the
other devices.
6. If you use RS-422 or RS-485 to connect just one HMT330 to a
master computer, enable the internal termination of HMT330 by
switching switches 1 and 2 ON. Make sure that the master's end of
the line is also terminated (by using master's internal termination or
with a separate terminator).
If you are connecting many transmitters to the same RS-485 bus,
make sure that switches 1 and 2 are OFF and terminate the bus with
separate terminators at both ends. This allows removing any
transmitter without blocking the bus operation.
NOTE If you use the internal termination of the transmitter at the end of the
RS-485 bus (instead of using separate terminators) removing that
transmitter will block the bus operation.
7. Use the bus type (4-wire/2-wire) to select the selection switch 3.
In 4-wire mode RS-485 master sends data to the HMT330 through
terminals Rx D1+ and Rx D0- and receives data from HMT330
through terminals Tx D1+ and Tx D0-.