RS-485 Unit Load
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TIA/EIA-485-A defines a unit load as a 15-kΩ resistor connected
to a –3- or 5-V source (see Figure 2). The –3-V case
applies for positive input current, and the 5-V case applies
for negative bus current. The definition and model are
valid for input voltages from –7 to 12 V to account for driver
outputs between 0 and 5 V, with up to ±7 V of commonmode
noise voltage between a driver and receiver.
The number of unit loads (nUL) presented by any proposed
connection to the RS-485 bus is then determined as
the ratio of its measured input current and the current of
1 unit load. Since the current of 1 unit load is a function of
voltage, the input current must be measured and the ratio
determined throughout the entire –7- to 12-V input-voltage
range, with the highest ratio determining the unit-load rating.
Figure 3 shows a hypothetical example where the measured
input current of a circuit is nonlinear. It can be
shown (see sidebar on next page) that, at its maximum
value, the ratio of the measured and unit-load current is
equal to the ratio of the slopes of the two functions and
lies on a line with an intercept at –3 V for positive current
and 5 V for negative current. Conceptually, this amounts to
rotating a line pivoted at I = 0 mA and V = –3 V until it is
tangent to the curve of measured positive current versus
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