Vaisala WXT510 - Manual (Page 44)

User’s Guide ______________________________________________________________________
42 __________________________________________________________________ M210470EN-D
With RS-485 and RS-422 interfaces, termination resistors should be
used at both ends of the line, if data rate is 9600 Bd or higher and
distance is 600 m (2000 ft) or longer. Resistor range 100 ... 180
Ω is
suitable for twisted pair lines. Resistors are connected across RX- to
RX+ and across TX- to TX+ (with RS-485 only one resistor needed).
The termination resistors will remarkably increase power consumption
during data transmission. If low power consumption is a must, a 0.1
μ F
capacitor should be connected in series with each termination resistor.
Note that the RS-485 interface can be used as well with four wires (as
the RS-422). The basic difference between the RS-485 and RS-422 is
actually their protocol. Namely, in the RS-422 mode the transmitter is
held constantly enabled, while in the RS-485 mode it is enabled only
during transmission (for allowing the host’s transmission in the two-
wire case).
The RS-232 output swings only between 0 ... +4.5 V. This is enough for
modern PC inputs.The recommended maximum for RS-232 line length
is 100 m (300 ft) with 1200 Bd data rate. Higher rates require shorter
distance, for instance 30 m (100 ft) with 9600 Bd.
Power Management
The power consumption of the WXT510 varies a lot, depending on the
selected operating mode or protocol, the data interface type, the sensor
configuration, and the measurement and reporting intervals. Lowest
consumption is achieved with the Native SDI-12 mode, typically less
than 1 mW in standby (0.07 mA @ 12 V), while with ASCII RS-232
or Continuous SDI-12 modes it is 2 ... 3 times higher. Any sensor
measurement, while being activated, adds its own extra consumption to
the standby power.
Some hints for economic power management are given below. The
current consumption values are all defined for 12 V supply. For 6 V
supply, multiply the values by 1.9. For 18 V or higher supply voltages,
multiply the values by 0.7 and add 1.5 mA for each 1 V increase above
18 V (see
Figure 14 on page 36 ).
NOTE When WXT510 is used on an RS-485 bus with other polled devices,
the error messaging feature shall always be disabled. This is done by
the following command: 0SU,S=N<crlf>.