But because the drive current is
Figure 7-10: Open-circuit biasing ensures a logic 1 logic state when the line is open
or no driver is active.
Designing RS-485 Links and Networks
129
much larger than the biasing current, the opposing bias current doesn??™t change
the logic level the receiver sees.
The biasing resistors and the parallel combination of the terminating resistors
and the receivers??™ inputs form a voltage divider. Smaller biasing resistors
increase the noise margin but also increase power consumption. With larger termination
values, the biasing resistors can be a little larger. A 3V supply requires
smaller values.
For an exact match, the termination at the node with the biasing resistors
should be slightly greater than the line??™s characteristic impedance, as in Figure
7-10. At the node with the biasing resistors, the terminating resistance equals
the terminating resistor in parallel with the series combination of the biasing
resistors. The function below accepts a desired termination resistance and a
value for the bias resistors and returns the value to use for the termination resistor
at the node with the biasing resistors. All values are in ohms:
Public Function TerminationResistor _
(ByVal desiredTerminationResistance As Integer, _
ByVal biasResistor As Integer) As Integer
TerminationResistor = _
(2 * desiredTerminationResistance * biasResistor) / _
((2 * biasResistor) - desiredTerminationResistance)
End Function
public Int32 TerminationResistor
(Int32 desiredTerminationResistance, Int32 biasResistor)
{
Int32 terminationResistorReturn = 0;
terminationResistorReturn =
(2 * desiredTerminationResistance * biasResistor) /
((2 * biasResistor) - desiredTerminationResistance);
return terminationResistorReturn;
}
For example, if desiredTerminationResistance = 120 and biasResistor = 620, the
function returns 133.
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