Section 5 Sensor Communication
24
FT742-SM (RS422 & RS485) Sensors
– User Manual
Figure 14: RS-422 Connection Diagram
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 chassis at each circuit node.
2. Surge protection is not shown.
3. Subject to testing, the bias resistors may be omitted, if the terminals RS422 converter makes use of a
fail-safe RS422 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 equation below.
Round the resistor up rather than down when selecting form 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
