Vaisala DMP248, Operating Manual

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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 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.
It can be mounted in the wrong direction or to the wrong pins without
breaking the module; it simply does not work then.
Network configuration
Single loop operation
Bi-directional data on one pair is one of the great advantages of the
RS 485 line. Set jumpers in connector X4 on the module board as
shown in the figure below.
X4
X4
X1 X2
This jumper setting connects RX HI to TX HI and RX LO to TX LO
and selects only one common line termination. The HI and LO
terminals of the RX pair can now be used for operation.
Supplying power from the same end to the whole network prevents
common mode voltages from rising too high (over 7 V).
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