The Bus Terminal And Bias Resistors - Danfoss VACON OPTE3 Installation Manual

VACON® RS485 and CAN Bus Option Boards
Installation Guide

5.4.2 The Bus Terminal and Bias Resistors

If the AC drive is the last device in the RS485 bus line, bus termination must be set. Use the jumper X13 (ON position) or external
termination resistors.
Bus biasing is required to make sure that communication between the devices in the RS485 bus line is faultless. Bus biasing makes
sure that the bus state is in the proper potential when no device is transmitting data. Without bus biasing, incorrect messages can
be detected when the bus is in idle state. The bus state of the RS485 bus line must not be between +0.200...+7 V or -0.200...-7 V. The
bus state <200 mV...-200 mV is illegal.
The resistances of internal termination and biasing are 120 Ω and 560 Ω.
+SV_1
560R
120R
560R
GND_1
RS-485 RX.TX
Illustration 21: Bus Termination and Biasing
If necessary, external termination and biasing can be added depending on the number of nodes and the total length of the cable.
Table 10: Bias and Termination Resistances
Number of nodes
2–5
6–10
11–20
21–30
31–40
If setting the jumpers does not give the desired outcome, add biasing resistors, termination resistors or both. For biasing with
OPTE2, connect the resistors between pins 2 and 3 and between pins 4 and 5. For termination with OPTE2, connect the resistor
between pins 3 and 4.
Danfoss A/S © 2022.08
X13-6
X13-4
X13-2
X13-5
X13-3
X13-1
Bias resistance
1.8 kΩ
2.7 kΩ
12 kΩ
18 kΩ
27 kΩ
RS485-based Option Boards
Termination resistance
120 kΩ
120 kΩ
120 kΩ
120 kΩ
120 kΩ
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