Vaisala HMP240 User Manual
Page 96
HMP240
Appendix 2: RS 485/422 serial port module
M210300en
94
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, only one line terminal
impedance is in use. The line must not be terminated with a resistor alone, as
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. If
mounted in the wrong direction or to the wrong pins, the module will not
break but it does not work.