Section 5 Sensor Communication
32
FT205EV Wind Sensors
– User Manual
RS422 is a multi-drop communication standard that permits network topologies containing one transmitter and
multiple receivers.
In RS422 configuration the default setting of the FT transmitter is to only enable while transmitting commands. A
software command (see Section 6.4.7) exists to activate the transmitter permanently, typically resulting in
additional current draw depending on network configuration. Termination and bias resistors generally improve
data integrity but may not be necessary if short cable lengths (0-
2m), low baud rates and when modern ‘fail-safe’
transceivers are used. The FT205EV uses
‘fail-safe’ transceivers. Removing the resistors may result in lower
current draw.
Figure 23: RS-485 Connection Diagram for 2 Wind Sensors
Notes:
1. Data lines A & B should be twisted-pair type (characteristic impedance 120
Ω). The cable should
incorporate overall screening braid which should be connected to the chassis at each circuit node.
2. Surge protection is not shown.
3. Subject to testing, the bias resistors may be omitted, if the terminals RS485 converter makes use of a
fail-safe RS485 transceiver.
4. Subject to testing, one or both termination resistors may be omitted only if biasing is
not
needed and the
transceivers are slew-rate limited.
5. Resistors should be of the anti-surge variety and rated at 1W or greater, such as the Tyco CCR range.
Rbias2 = 750
Ω, Rterm = 120Ω, see table below for Rbias1 values at various voltages (E24 series). If
operating at a voltage other than what is specified in the table, select Rbias1 based on the equation below.
Round the resistor value up rather than down when selecting from the E24 range.
𝑅𝑏𝑖𝑎𝑠1 (𝑉𝑠) =
𝑉𝑠 × 754
3.125
− 754
Supply Voltage
(V)
Rbias1
(
Ω, E24 5%)
6
715
12
2200
24
5100
30
6800
