10 • vacon
grounding cable shield
Tel. +358 (0)201 2121 • Fax +358 (0)201 212 205
4
4.4
Bus Biasing
Bus biasing is required to ensure faultless communication between devices at RS-485 bus. Bus
biasing makes sure that the bus state is at proper potential when no device is transmitting. Without
biasing, faulty messages can be detected when the bus is in idle state. RS-485 bus state should be
n0,200..+7V or –0,200..-7V. Illegal bus state is <200mV..-200mV.
Number of nodes
Bias resistance
2-5
1.8 kohm
5-10
2.7 kohm
11-20
12 kohm
21-30
18 kohm
31-40
27 kohm
Table 4-1. Bias resistor size vs number of node
Fail safe
biasing in OPT-CJ option board
Connect resistor biasing resistors between pins #2 and #4 as well as pins #3 and #5 as shown in
picture.
Matters related to this are discussed in the application note
Failsafe Biasing of Differential Buses
(an-847.pdf) published by National Semiconductor (
www.national.com
).
5
DATA A
DATA B
1
2
3
4
5
