Section 7
Parameter Settings
40
FT742-SM (RS422 & RS485) Sensors
– User Manual
7.3 Timing Constraints
When a valid command is received by the sensor input buffer, there will be a time delay whist the command is
being processed. The actual command latency depends on exactly when the last character of the command is
received within the sensor internal processing cycle. The sensor can process only one SET or QUERY command
at a time.
Figure 28: Command Processing
Once a SET command has been received by the sensor, it can take approximately 400ms for the command to be
processed and any setting change implemented. If other commands are sent during this period, they may be
ignored by the wind sensor. Therefore all SET commands must be separated by a period of at least 500ms before
further commands are sent. (Or longer in the case of a Reset command being sent See Section 7.4.20).
Once a QUERY command has been received by the sensor, it takes up to 50ms for the command to be processed.
The sensor will then wait for a predefined delay before sending a response. This delay time is programmable in
increments of 50ms.
See Section 7.4.10 for details of how the delay between the sensor receiving a command and transmitting a reply
can be adjusted. An additional latency is recommended depending on the time taken for the computer to switch
between transmit and receive modes.
If the delay time has been extended using the DL command, then the next QUERY command sent to the wind
sensor must be separated by the maximum QUERY command processing time (50ms) and at least the DL interval
(50ms default.) It is therefore recommended that the frequency of any QUERY command does not exceed 10Hz,
i.e. 10 commands per second.
Message Example:
For example, to set the wind velocity output to Polar format and verify that the command has been accepted, send
the following commands:
Set the wind reading format to polar:
$//DFP*//<cr><lf>
Then wait 500ms for the SET command to be implemented by the sensor.
A QUERY can then be sent to confirm command has been carried out:
$//DF?*//<cr><lf>
Then wait 50-100ms for the sensor to send a response:
$WI,DF=P*5D<cr><lf>
Please note the above example assumes the sensor has a factory default time delay of 50-100ms (DL01).
