Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 32 of 137 Manual Documentation Number: <DocNumber>
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AC Termination
AC coupled termination adds a small capacitor in series with the
termination resistor to eliminate the DC loading effect, which affects
biasing. Although this method eliminates DC loading, capacitor
selection is highly dependent on the system properties. System
designers interested in AC termination are encouraged to read National
Semiconductors Application Note 903 (note 2) for further information.
Figure 22: AC Termination
Termination to Reduce Noise
In some instances where termination would not otherwise be needed, if
noise pickup on the communications line is a problem, it may still be
necessary to add a termination resistor. In this case adding a
termination resistor with a value larger than the line impedance--for
example 1 k may reduce noise while still maintaining a usable bias
voltage. The primary consideration is that at least 300 mV should be
maintained across the signal lines while the line is idle.
Biasing an RS-485 Network
When an RS-485 network is idle (no data being transmitted) all RS-485
drivers are inactive, in the high impedance state (tri-stated). Receivers
are enabled, listening for the next data message. During this time the
communications line should be in the idle state (mark, logic 1,V
AB
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