Vaisala WXT520 - Manual (Page 154)

User’s Guide ______________________________________________________________________
152 _________________________________________________________________ M210906EN-C
Example (composite data message queries to the sensors 1 and 3 are
assigned as follows):
0R0<cr><lf>
1R0<cr><lf>
2R0<cr><lf>
NMEA 0183 v3.0, Query
The NMEA 0183 query messages do not contain device address
information. Individual query commands can thus not be directed to
different transmitters. Instead, a specific Time Slot method can be used
for receiving data from several transmitters on the bus, just with a single
query command.
To generate different time slots, each WXT520 is given an individual
delay for its query response, by using the RS-485 line delay parameter
aXU,L. This parameter defines the time (in milliseconds) between the
last character of the query and the first character of the response from
WXT520.
Example (a bus with three WXT520s):
WXT520 #1 communication settings:
0XU,A=0,M=Q,C=3,I=0,B=4800,D=8,P=N,S=1,L=25
WXT520 #2 communication settings:
0XU,A=0,M=Q,C=3,I=0,B=4800,D=8,P=N,S=1,L=1000
WXT520 #3 communication settings:
0XU,A=0,M=Q,C=3,I=0,B=4800,D=8,P=N,S=1,L=2000
Now, when the XDR-query command $--WIQ,XDR*2D<cr><lf> is
sent, WXT520 #1 responds after 25 ms, WXT520 #2 after 1000 ms and
WXT520 #3 responds after 2000 ms. The sufficient delays depend on
the maximum number of characters in the response messages and the
baud rate. Note that all the transmitters are assigned with the same
address. Hence the data logger, after sending the query, shall sort out the
response messages on the basis of the individual response times.
To gain even more addressability transducer ID information provided in
the XDR response messages can also be used. If WXT520 address is set
to 0 (aXU,A=0) and all parameters are chosen except Rain peak