Specifications

Chapter 2 - System Configuration RS-422 And RS-485 Applications Ebook
Page 35 of 137 Manual Documentation Number: <DocNumber>
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Current through the voltage divider is approximately 5 mA.
The voltage across the signal lines (V
AB
) is -285 mV. Although slightly
less than the goal of -300 mV, this is acceptable for idle state biasing.
Figure 24: 10 Node RS-485 Network with two 120 ohm termination resistors
Notice that the current is relatively high and the termination resistors
are responsible for a majority of the loading. Adding nodes with built-in
biasing resistors would increase the minimum idle state voltage but
would further increase the current. Decreasing nodes would eventually
result in insufficient idle state voltage. Ultimately, the example shows
that the combined effects of termination and idle state biasing must be
calculated and biasing resistance adjusted, if necessary to ensure
reliable idle state performance.