Young Ultrasonic Anemometer Voltage and Serial Output Model 81000 - Manual (Page 5)

Page 4
81000-90
CHKSUM calculates the bitwise exclusive-or of all characters in
the output string starting with the rst character and ending with
the last character of the last eld before the checksum. The check-
sum is expressed as a two-digit hex value preceded a space.
ERROR CODE indicates the validity of the measurement. Any
non-zero value indicates an invalid measurement. ERROR
HANDLING must be set to INCLUDE INVALID DATA to use
this eld. Keys to the error codes provide no useful informa-
tion to the user.
INTERNAL VOLTAGE is the internal supply voltage. Because
it is measured after current overload protection devices, it will
always be less than the supply voltage measured at the con-
nection terminals.
NMEA
NMEA sends wind speed and direction in NMEA marine format to
Young Model 06206 Marine Wind Tracker display or other NMEA
device. The sentence is $WIMWV,aaa,R,ss.s,N,A where aaa =
wind direction angle in degrees and ss.s = wind speed in knots.
When NMEA is selected the OUTPUT RATE is changed to 4 Hz
and THRESHOLD is set to 50 cm/sec. Most NMEA systems use
4800 baud but, because some systems use other baud rates, this
setting should be changed manually if necessary.
POLLED CUSTOM
POLL CUSTOM format allows the sensor to be polled for a mea-
surement. See CUSTOM for details on constructing the output
string. Poll by sending MA! (where 'A' is the POLL CHARACTER
ADDRESS).
After a measurement has completed, the sensor responds with the
POLL CHARACTER followed by the custom serial output string.
Up to 32 sonic anemometers may be networked using the RS-485
connection. By assigning a unique address to each device, mul-
tiple units may run on the same network and respond individually
only when polled.
POLLED BINARY
POLLED BINARY starts a measurement and sends serial data in
binary rather than ASCII format. The advantage of using POLLED
BINARY output format is that data output strings are shorter
and data does not have to be parsed and converted from ASCII
characters to numeric values. The disadvantage is that the output
cannot be examined directly as text. The 10-byte binary output
string description appears below.
Byte
Index Description
0 U vector cm/s (signed 2-byte integer)
2 V vector cm/s (signed 2-byte integer)
4 W vector cm/s (signed 2-byte integer)
6 T Sonic temperature K x 100 (unsigned 2-byte integer)
8 Status (byte non-zero = error
9 Checksum (byte, XOR of all char, hex val)
The POLLED BINARY format may be employed using one of two
different methods.
The rst method uses the MA! command to initiate a sonic
measurement ('A' is the sensor POLL CHARACTER ADDRESS,
see section 6.6). When the measurement is complete, the sensor
automatically transmits measurement data in binary form. The
delay between the start of the measurement and data availability is
determined by the OUTPUT RATE parameter (see section 6.5)
The second method uses separate trigger and data-retrieval
commands. Any sensor that receives a Tr! command while set
for POLLED BINARY operation will immediately start a sonic
measurement. The command is not address-speci c. When the
measurement is complete, data will be available for retrieval using
the RA! command ('A' is the unique POLL CHARACTER AD-
DRESS, see section 6.6).
The time required to complete a measurement after the Tr! com-
mand depends on OUTPUT RATE. Also, if multiple sensors are
connected on a shared RS-485 bus, each RA! command and its
reply must be given enough time to complete. The time constraints
are shown in the table below:
Output Measurement Minimum Delay between
Rate (Hz) Complete (ms) bussed RA! cmds (ms)
4 225 33
5 175 23
8 115 13
10 90 11
16 58 8
20 45 8
32 25 8
The Tr!/RA! trigger/retrieve method is useful for synchronizing
simultaneous measurements with multiple 81000 sensors. After
completing a measurement triggered by the Tr! command, data
from each sensor (uniquely addressed) can be retrieved with the
RA! command. Data rates up to 10 Hz with six sensors on a single
full-duplex bus have been demonstrated (38.4k baud). Faster
rates or more sensors may be con gured by adjusting operating
parameters.
6.8 THRESHOLD
THRESHOLD sets wind speed threshold for the following output
formats only: NMEA, SPEED AZIMUTH ELEVATION. UVW out-
puts are unaffected. Setting THRESHOLD greater than 0 prevents
uctuating azimuth and elevation values at wind speeds near zero.
While these values are correctly derived, they are meaningless or
distracting in some applications.
When wind speed is at or above threshold, all output values are
updated normally. When speed is below threshold, speed and
elevation outputs are set to zero and the azimuth output is held at
its last value. Setting THRESHOLD to 0 effectively disables the
feature. Note that units are cm/sec.
THRESHOLD (cm/s) : 20
ENTER NEW THRESHOLD (cm/s, 0-500) :
6.9 UNITS
UNITS sets wind speed units for CUSTOM serial output. Resolu-
tion associated temperature units are as follows:
Wind Speed Temperature
units and resolution units and resolution
cm/s 1 K 0.01