12 • vacon layout and connections
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3.5 Bus Biasing
Bus biasing is required to ensure faultless communication between devices at RS-485 bus. Bus bias-
ing makes sure that the bus state is at proper potential when no device is transmitting. Without bias-
ing, faulty messages can be detected when the bus is in idle state. RS-485 bus state should be
neather +0,200..+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. Bias resistor size vs number of node
Fail safe
biasing in OPTC2 option board
Connect resistor biasing resistors between pins #2 and #4 as well as pins #3 and #5 as shown in pic-
ture.
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).
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