Vaisala WMT52 - Manual (Page 126)

User’s Guide ______________________________________________________________________
124 __________________________________________________________________M210925EN-B
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 devices. Instead, a specific Time Slot method can be used for
receiving data from several devices on the bus, just with a single query
command.
To generate different time slots, each WMT52 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
WMT52.
Example (a bus with three WMT52s):
WMT52 #1 communication settings:
0XU,A=0,M=Q,C=3,I=0,B=4800,D=8,P=N,S=1,L=25
WMT52 #2 communication settings:
0XU,A=0,M=Q,C=3,I=0,B=4800,D=8,P=N,S=1,L=1000
WMT52 #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, WMT52 #1 responds after 25 ms, WMT52 #2 after 1000 ms and
WMT52 #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 devices 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 WMT52 address is set
to 0 (aXU,A=0), the response to the XDR-query
$--WIQ,XDR*2D<cr><lf> will be like:
$WIXDR,A,316,D,0,A,326,D,1,A,330,D,2,S,0.1,M,0,S,0.1,M,1,S,0.1,
M,2*57<cr><lf>
$WIXDR,C,24.0,C,0,C,25.2,C,1,H,47.4,P,0,P,1010.1,H,
0*54<cr><lf>
$WIXDR,V,0.000,I,0,Z,10,s,0,R,0.01,I,0,V,0.0,M,1,Z,0,s,1,R,0.0,M,
1*51<cr><lf>