Vaisala HMP230 - Manual (Page 115)

HMP230 SERIES
M210225en-A Appendix 3: RS 485/422 serial port module
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The RS 485/422 module has separate lines for transmitting and receiving, but
they can be connected together with jumpers. Dual loop connection is the
factory setting; when a single loop connection is used, the positions of jump-
ers in connector X4 on the module must be changed.
The HI of the receiving line is approx. 0.6 V and its LO is approx. 0 V in or-
der to reduce noise on the lines when no data is transferred (idling). Both lines
are terminated with a 120 ohm resistor in series with a 33 nF capacitor. When
operating the transmitter through a single pair, naturally only one line
terminal impedance is in use. The line must not be terminated with a resistor
alone, as then the power consumption increases too much.
The data lines can withstand short circuit to ground and to each other. They
do not survive connection of supply voltage to the data lines.
The module must be mounted on the main board in the right direction. It can
be mounted in the wrong direction or to the wrong pins without breaking the
module; it simply does not work then.