Vaisala HUMICAP HMT330 SERIES Uživatelská příručka Strana 58

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User's Guide ______________________________________________________________________
58 __________________________________________________________________ M210566EN-H
Table 6 Connecting the Twisted Pair Wires to the Screw
Terminals
Screw terminal Data line
(2-wire RS-485)
Data line
(4-wire RS-485/422)
1 (not connected) RxB
2 (not connected) RxA
3 Data pair shield Data pair shield
4 B TxB
5 A TxA
6. If you use RS-485 (or RS-422) 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 RxA and RxB and receives data from HMT330 through
terminals TxA and TxB.
Figure 37 4-Wire RS-485 Bus
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