Vaisala HMP260 - Manual (Page 93)

HMP260 SERIES
Appendix 3: RS 485/422 serial port module
HMP260-U017en-1.5
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
jumpers 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
order 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. The
module will not break if it is mounted in the wrong direction or to the wrong
pins but it does not work.