Section 5
Sensor Communication
28
FT702LT Flat Front Wind Sensor Manual
5.3.4
Listener and Talker Identifiers
The FT702LT is assigned with both a Listener and Talker identifier address that allows an individual FT702LT to
be uniquely identified in a system comprising more than one FT702LT.
Whenever a message is sent to the FT702LT, the identifier field of the message (the 2 characters immediately
followi
ng the ‘$’ start of message character) must correspond to the FT702LT Listener identifier address,
otherwise the FT702LT will ignore the message. In applications where more than one FT702LT is connected to
the RS485 bus, you should assign each FT702LT in the system a unique Listener ID. The host computer will
then be able to address individually each FT702LT. If you do not wish to use the Listener ID in messages sent
from the host computer
, you can replace the Listener ID with ‘//’. Sending ‘//’ in place of the Listener ID will allow
any FT702LT, irrespective of its Listener ID setting, to respond to the message.
Whenever a message is transmitted from the FT702LT, the identifier field of the message (the 2 characters
immediately following the ‘$’ start of message character) will contain the Talker ID. The Talker ID is used as a
message tag to identify which FT702LT has transmitted the message.
The factory default value for the Listener ID is 01 and for the Talker ID it is WI (Weather Instrument). To change
the Listener and/or Talker ID use the ID Command, Section 6.4.13.
5.3.5
Calculating the Message Checksum
All messages sent to, or received from, the FT702LT include a checksum field. Messages that are transmitted
from the FT702LT always include a checksum value in the checksum field. Messages sent to the FT702LT by
the host computer
can either contain a checksum value or an ‘ignore checksum identifier’ in the checksum field.
Hex Values
50
46
44
2C
32
30
XOR Result:
7C
$
2 , D F P * 7 C <cr> <lf>
0
Characters used to
form Checksum
Figure 26: Checksum Example
The checksum value is calculated by Exclusive OR’ing (XOR’ing) all the bytes between (but not including) the
‘$’ and the ‘*’ characters of the message. The resulting single byte value is then represented by 2 HEX
characters in the message string. The most significant character is transmitted first.
Note: since a message only contains ASCII characters (which have values in the range 0-7F) the checksum
value will always be between 0 and 7F.
5.3.6
Disabling the Checksum
All messages which are sent to the FT702LT must contain a valid checksum value in the checksum field,
otherwise the FT702LT will not process the incoming message. Although it is recommended that a checksum
value be computed for all messages which are sent to the FT702LT, in some cases this may not be convenient
(i.e. when communicating with the FT702LT with a terminal). To prevent the FT702LT from performing
checksum validation of incoming messages, send the ASCI
I characters ‘//’ in place of the checksum value.
Example:
Send a message to set the data output format to Polar using the DFP command (the FT702LT Listener ID in
this example is set to 02)
