Section 3
Installation
13
FT702LT Flat Front Wind Sensor Manual
3.1.3
RS-485 Protocol
The FT702LT is fitted with an RS485 half-duplex serial interface. Slew-rate limited drivers are used to reduce
EMI, and minimize reflections from improperly terminated transmission lines and stubs. The signal state
definitions for the serial interface data lines are as follows:
-
T
he idle, marking, logical “1”, OFF or stop bit state is defined by a negative voltage on line A with
respect to line B.
-
T
he active, spacing, logical “0”, ON or start bit state is defined by a positive voltage on line A with
respect to line B.
Figure 6 shows the recommended wiring diagram for connecting the FT702LT to the computer.
If two or more wind sensors are to be installed it is possible to use the same 2-wire data link to connect all the
FT702LT units to the computer.
Before using an FT702LT in a multi-device system, the Listener identifier of each FT702LT must be set to a
unique value. Use the ID command (Section 6.4.13) to set the Listener identifier for each FT702LT. If the
Listener identifiers are being set in the final host system, then it is important that only one FT702LT be
connected at a time to the RS485 bus until all devices have been assigned a unique Listener ID.
Great care should be exercised when using the ‘//’ characters for addressing. The ‘//’ address characters can be
used to send a SET command simultaneously to all FT702LT units (for example, to enable or disable filtering).
Under no circumstances should the ‘//’ characters be used with any QUERY commands since this will cause all
FT702LT units to transmit data resulting in bus contention.
Figure 6: RS-485 Connection Diagram for 2 Wind Sensors
