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Keywords: differential signal detection, RS485, window comparator, loss of signal
APPLICATION NOTE 6009
DETECT A DIFFERENTIAL SIGNAL EVEN WHEN
THE INPUTS ARE STUCK
By: Budge Ing, Applications Engineer, Maxim Integrated
Abstract: It is desirable to know if the incoming differential signal is present to notify a monitoring device of
the loss of signal. This app notes describes a simple circuit for monitoring for these conditions and reporting
failures.
It is advantageous to know if there is a valid signal on a differential signal bus. This design idea detects
differential data transmission and notifies a monitoring microcontroller about any loss of signal (LOS).
The circuit of Figure 1 detects a continuous differential signal from -7V to +12V with a minimum amplitude
difference of 200mV. The circuit detects open inputs, shorted inputs, and inputs stuck at high or low. Only a
single 5V supply is needed. The circuit's output is low when any one of these conditions occurs; the circuit is
high when there is a pulse train of the differential signal.
U1 (MAX3280E), an RS-485 receiver, accepts differential signals of 200mV minimum at A and B, where the
voltage can be from -7V to +12V. RO of U1 is high in the absence of input signals, because U1 is a true fail-
safe device that provides a high output when the inputs are open or shorted. RO of U1 can also be high or
low when one of the inputs is stuck in high and the other is stuck in low. R1 and C1 form a lowpass filter to
rectify the output at U1 into a DC voltage when the output at RO is a pulse train.
U2 (MAX992), a dual open-drain comparator, is configured as a window comparator with R2, R3, and R4
setting the input voltage thresholds to 0.5V and 4.5V. The filtered voltage of a pulse train falls between the
two thresholds rendering OUT as high.
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