Section 5 Sensor Communication
22
FT742-SM (RS422 & RS485) Sensors
– User Manual
6 SENSOR COMMUNICATION
6.1 Introduction
The sensor features an easy to use ASCII-based communication protocol transmitted over either RS422 (full-
duplex) or RS485 (half-duplex) serial link. The sensor is configured to either RS422 or RS485 in the factory (this
setting cannot be modified by users). The protocol incorporates checksum validation to ensure the integrity of all
data transmissions. In addition to the FT Technologies proprietary protocol the sensor can output the common
NMEA 0183 MWV (Wind Speed and Angle) sentence.
6.2 RS422 & RS485 Protocol
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:
- RS485
The idle, marking, logical “1”, OFF or stop bit state is defined by a negative voltage on line A with
respect to line B.
- RS485
The active, spacing, logical “0”, ON or start bit state is defined by a positive voltage on line A with
respect to line B.
- RS422 -
The idle, marking, logical “1”, OFF or stop bit state is defined by a negative voltage on line A
with respect to line B.
- RS422 -
The active, spacing, logical “0”, ON or start bit state is defined by a positive voltage on line A
with respect to line B.
Circuit examples are provided in Figure 13 and Figure 14.
If two or more wind sensors are to be installed, it is possible to use the same 2-wire RS485 data link to connect
all the sensor units to the computer.
Before using a RS485 configured sensor in a multi-device system, the Listener identifier of each sensor must be
set to a unique value. Use the ID command (Section 7.4.13) to set the Listener identifier for each sensor. If the
Listener identifiers are being set in the final host system, then it is important that only one sensor 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 sensor 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 sensor
units to transmit data resulting in bus contention.
RS422 is a multi-drop communication standard that only permits one FT742-SM sensor to be connected.
