Section 5 Sensor Communication
27
FT742-SM (RS422 & RS485) Sensors
– User Manual
6.4.4
Listener and Talker Identifiers
The sensor is assigned with both a Listener and Talker identifier address that allows an individual sensor to be
uniquely identified in a system comprising more than one sensor.
Whenever a message is sent to the sensor, the identifier field of the message (the 2 characters immediately
following the ‘$’ start of message character) must correspond to the sensor Listener identifier address,
otherwise the sensor will ignore the message. In applications where more than one sensor is connected to the
RS422/RS485 bus, you should assign each sensor in the system a unique Listener ID. The host computer will
then be able to address individually each sensor. If you do not wish to use the Listener ID in messages sent
from the host computer, you can replace the Listener ID w
ith ‘//’. Sending ‘//’ in place of the Listener ID will allow
any sensor, irrespective of its Listener ID setting, to respond to the message.
Whenever a message is transmitted from the sensor, the identifier field of the message (the 2 characters
immediate
ly following the ‘$’ start of message character) will contain the Talker ID. The Talker ID is used as a
message tag to identify which sensor has transmitted the message.
The factory default value for the Listener ID is 01 and for the Talker ID it is WI (W eather Instrument). To change
the Listener and/or Talker ID use the ID Command, Section 7.4.13.
6.4.5
Calculating the Message Checksum
All messages sent to, or received from, the sensor include a checksum field. Messages that are transmitted
from the sensor always include a checksum value in the checksum field. Messages sent to the sensor 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 18: 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.
6.4.6
Disabling the Checksum
All messages which are sent to the sensor must contain a valid checksum value in the checksum field,
otherwise the sensor will not process the incoming message. Although it is recommended that a checksum
value be computed for all messages which are sent to the sensor, in some cases this may not be convenient
(i.e. when communicating with the sensor with a terminal). To prevent the sensor from performing checksum
validation of incoming messages, send the ASCII characters ‘//’ in place of the checksum value.
Example:
Send a message to set the data output format to Polar using the DFP command (the sensor Listener ID in this
example is set to 02)
