Answer / carl_ellis

You are looking at the RS-485-side of an RS-232/RS-485
converter and the power supply voltage for the converter is
+5V. Under idle conditions (no transmission), and assuming
you have enabled the RS-485 output: what would you expect
at: (a) Output A measured against output B (b) Output A
measured against ground (c) Output B measured against ground

With a supply voltage of 5V, the sink driver is usually able
to pull the voltage on each conductor fairly close to signal
ground (0.5V), but the source driver usually has a voltage
drop of approximately 1-1.5V. The voltage swing on each
conductor is therefore typical 3-3.5V, but it may be as low
as approximately 2V.

Answer a)
Hence, output A (-) measured with respect to output B (+)
would read approx -3V to -3.5V ( a negative voltage) in the
idle state. (The logic is inverted: a negative voltage is a
mark or logic 1)

Answers b and c)
Output A (-) measured with respect to ground: ~3.5V to 4V
Output B (+) measured with respect to ground: ~0.5V to 1V

This is a 3V differential, with 2.5V Vcm common mode, where
common mode is the mean of the differential plus the Vcm
common mode above the reference ground.

Carl Ellis
Measure First

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